Thanks for your questions. I think this questions ” Why can’t applications be developed on mobile phones?” is very important for all like yours.Here I’m trying to give your answers in brief.

Theoretically they could, but there is a number of problems.
The big problem is the lack of a keyboard. Yes you have a soft keyboard on most mobile devices, and a few have a hardware keyboard, but think about the effort necessary to type even one page of text on a mobile device compared to a normal computer.
Screen space – While developing applications, being able to see more at one time makes the process more efficient (especially if half of that screen space is taken up by the soft keyboard).
A related problem is that you probably will be wanting to look some things up on the web while solving some of your problems – Uses up more screen space.
Testing – Ideally you would like to test your completed app on a wide variety of devices. If you develop on the device, you will still have to copy it to other devices, or have emulators on your device.
Speed – Most mobile devices are designed to use battery efficiently, and mobile devices tend to have slower CPUs than desktops (As an example, a sort that takes about 5 seconds on my PC takes over 36 seconds on my mobile phone).
Storage – Mobile phones tend to have 8–200 GB of storage (depending on the device and SD card) while my PC has over 400 GB. Given that my Android Studio folder is over 46 GB, a mobile device would be limited.
Software – I am not aware of any development environments that run on mobile devices, but this is mostly due to the other problems listed above.

Mobile applications are built for two platforms, Android and IOS operating systems.
In the development of IOS apps, companies use “Xcode” integrated development environment and “Swift” programming language. Whereas “Android studio” IDE and “Java” programming language are used in Android app development.
Coming to your question, it is not possible to build a mobile app on mobile itself. So the reason, why we can’t use mobile as a platform for app development,t is
1. Hardware and software – Mobile phone is not compatible with softwares that are required to develop an app.
2. Small screen – A mobile app development company provides big screen computers to their developers to boost their productivity. Typing on a small screen like a mobile device will make it hectic for programmers to code.
3. Speed – Processors and operating system of computers are quite faster and handy than of mobile.
4. Mobile phones are not designed for compiling huge complex codes.
In short, it is impossible for now in 2018.
Maybe it could become possible in future as the technology is evolving very fast.
Hope the answer was helpful.