This is not limited to CS or Latex in any way. Plenty of students spend a lot of time fiddling with word, powerpoint, note taking systems, citation management (which is surprisingly horrible in MS word), Adobe software etc..

Obvious reasons:

- Your thesis is a major output of years of work. Of course you want it to look good.

- You might think it superficial, but if the presentation looks bad, many people (subconsciously) interpret this as a lack of care and attention. Just like an email with typos feels unprofessional even if the content is otherwise fine.

- Spending time on tooling feels productive even if it is not past a certain point.

- People that are into typesetting now have an excuse to spend time on it.

That said, in my experience people spent a few hours to learn "enough" latex several years ago and almost never write any macros. Simple reason: you work with other people and different journal templates, so the less custom code the better.