When you are the startup you dont have resources on tests. Cause tests is basically x2-x3 for resources. When you are the startup you need move fast, cause time & money is more valuable the code quality and test updates are freezing you.
And general thing about testing — in most cases it not prevent you from real bugs in production, it’s just remove some simple bugs. And if you will try to make testing faultless then you will end up in some NASA team, where is testing taking 99% of all development time and multiply resources from x25. And still not prevent on 100% from bugs.

Also you should know difference between school and real life, cause it’s the main thing.