Regarding the tone of this thread, part of what's going on appears to be a small culture clash. The long-time WP users were drawn to the OS early and in part by its simplicity, despite missing many familiar apps. Now that the app gap is being filled in, new users are coming to seek something new. The new-comers are asking for features that they had before and the long-timers don't want to lose the simplicity that drew them to WP initially.
The number of requests for live battery meters, notification centers, and quick-toggle buttons for radios is increasing. On the bright side, it means that more users are coming and new features will follow them. On the down side, it means that possibly unwelcome change is coming. I trust that the folks who designed WP had some key principles in mind and won't lose them. They also likely know what a mess WM6 had become and won't be eager to repeat it.
To the OP, several folks have answered this thread's title question: because Microsoft believed in both simplicity and that people use phones as appliances rather than computers. Microsoft wants you to be a "user" of your phone, not the "system administrator" of it. You may not like that philosophy, but complaining here won't change it, although suggesting improvements at windowsphone.uservoice.com might over time. (Or it could be that they just copied the battery indicator from Windows 7.)
That said, there are lots of ways to solve problems. Microsoft could copy its own implementation for status icons near the clock of the regular Windows desktop. (The status bar in WP and the area near the clock of regular Windows are both called the System Tray.) That lets users choose which icons appear all of the time and which ones appear only when they have something significant to say. If MS built some sort of Status Bar Settings page, we could each choose which icons should we treated like the clock (i.e., always visible when the running app lets the tray show) and which should be hidden unless called upon (by tapping the tray) and the battery indicator could have a toggle for "iconic/percentage".
I always thought the reason why MS didn't show a percentage indicator was because it didn't fit in with their metro/modern design vision and the fact that the status bar disappeared was to help create that clean, simple look and feel of Windows Phone.
To be fair, my Nexus 5 running KitKat doesn't show the percentage by default either. I either have to swipe down and check via quick settings or install a 3rd party app just like on Windows Phone.
A good app for WP is called Battery Pro+. I think it's the only one that shows a 100 on the tile.
These two posts give all the answers anyone moaning about battery percentage should need.
It is totally different to what "you need" or what "you think MS should make WP have" - but if you have been here long enough, especially during the transition from WM6.5 to WP7.0, you would know what WP was supposed to be. More users ranting about notification centre and toggle drop downs are clearly those migrating from other slower OS' phones because they thought or were told WP was slim and fast. I completely agree, that absence of disgusting percentage sign from the status bar and absence of status bar itself was to compliment the gutter on the right for WP7. The reason being WP7 was aesthetically pleasing. I am one of those few hundreds out here who did not like removal of gutter (we have massive threads on it if you search). The reason being design philosophy of WP was
to have information coming to you when it has to. Not constantly nagging you.
WiFi icon appears when phone is trying to connect, appears when you are using WiFi so that you know your mobile data is not being used. Disappears when its off - you don't need to know its off. However, greys out when you lost the connection.
Ringing bell icon appears when you put phone on silent or changes to vibrating phones when its on vibrate. In normal circumstance - the phone doesn't shove it on your screen to tell you that your phone can ring!
Same theory - with
network appears (drops down literally!) to tell you you've lost network! Otherwise if you are connected, it doesn't need to keep shoving those bars in your face and waste the clean Modern look. The
battery icon - appears when you have less than 20% battery to tell you this is when you need to be careful - otherwise as long as it doesn't appear you have enough juice. At 20% you can even have battery saver automatically kick off and you know - that will last you twice your normal time!
That is what WP does and does beautifully. But as told earlier by users, new folks join the crew, they want Android that is faster. They want Android that has Xbox points and they want Android that has Office docs and an Android that will sync to their home PCs so in the next few iterations we will get that Android. We will end up having a file explorer, a quick toggle, a notification centre, a live wallpaper, folders etc - it will be an Android. Why? Because MSFT is in to earn money, not win design awards. It will see that people don't get the whole "Modern design" philosophy and it will kill it before we know it. We will say its "adaptation" or "evolution" and move on. MSFT has always had things before time and suffered. Flat design was there since 2010 but iPhone has it in 2013 and all of a sudden Flat design is in! Can we not see even Nexus is following the trend and removing those features and going flat like Metro of 2010?? No.