Why don't people write their own servers?

Unity Engine
1

I don’t understand why people are using things like photon cloud.
Is it not much better and easier to write your own server?

What is so appealing about these programs?

2

Maybe having something stable and ready to use, without requiring x amount of time spent in writing the code for networking.

3

I don’t understand why people are using things like Unity.
Is it not much better and easier to write your own game engine?

What is so appealing about these programs?

Same thing?