Well... your current employer might let you go or freeze your upward mobility if they find out you are looking at other jobs.

I've even seen this internally. If I want to go to another team, why would my manager use their budget or political influence to promote me when they can use it on someone who is staying on their team.

This is why you do not tell your manager until it's decided, or it's so far along that they're needed as a reference.

In a perfect world, your manager should not care and your company should not treat you any differently for looking, but this is rarely the case.

Externally, I agree. On a side note, my company will not provide references when you leave and forbids managers from providing any references.

At my company, internal postings notify your manager when you apply, so there's no avoiding it. It's generally better to talk to them first so they aren't blindsided by the posting. Otherwise, if you don't get the new position that could create some friction with the existing one. My company can also trap you in your current position if they designate you as a "key resource".

You'll never pass if you dont prepare, this is bad advice

If you spend so much time preparing that you never interview, you'll never pass either.