Why is it that every time MPs don't like something, they resign?
How does this help whatsoever? They lose their job and therefore their influence to actually fix the issue they're so invested in.
Sure, it draws attention to the problem, but it does not help to fix it.
Am I missing something?
A cabinet MP cannot vote against their government's wishes and retain their position as a cabinet minister. So, if an MP feels that the government's wishes no longer align with their own, they have a simple choice: abandon their principles and keep their position, or resign from the government in order to vote against the motion(s) in parliament, becoming a backbencher in the process.
Resigning from the government can also be a case of simply disagreeing with the government's general position rather than being about a single vote, which is what we've seen today in the whole Brexit thing: government ministers no longer seeing eye-to-eye with May on Brexit, and in some cases do not want May to remain as PM at all. It would be a conflict of interest for them to remain a part of the government if they do not agree with that government's wishes.
Sometimes resigning is the best mechanism to exert their influence.
Take Tracey Crouch - last week she resigned because the government had delayed their promise to tackle fixed odds betting stakes. Before the end of the week the government returned to their initial plan - what Crouch wanted - because of the attention (and criticism) she brought to the issue.