As a teacher, I can tell you that, it is simply just that as you said - to have something to put on the portfolio. Remember that, for students, they have zero experience. Therefore, they aren't capable of addressing the needs or find the real underlying problem, and finally come up with a great solution.

For young designers, the more stuff they make and show to the people, the bigger the chance someone will notice them. How to start if you don't have client work yet?

The starting point of learning art and design has always been copying. Rebranding a famous brand or redesigning any UI is a shortcut to learn how to create a comprehensive project. But students and as well self-taught designers tend to forget that it's just the beginning. The next step would be create a total conceptual/fictional brand or app.

Why now bunch of designers doing wikipedia? Because it's one of the easiest website out there to uplift the styling. It's the website that has millions of users - meaning every viewer of the portfolio would know what wikipedia is. It's also for an easier for potential client to spot good/bad design (although there's a lot of clients who go with eye candy design). What these wikipedia redesigners and dribbblers aren't aware is that, this is a step backward if they already have real clients. Student projects and fake redesign should be buried as soon as they've got real clients. These redesign should be for practice only, not to further marketing yourself and get web traffic or likes.

Although I kinda see their perspective of wikipedia is truly free and open for users to collab and edit (even with UI design), it's sad that even wikipedia itself support these unsolicited design. It just keep these non-sensical design going on.

I read a lot of helpful comments here, but by reading yours, I have the feeling, that you have no patience to look int the screenshots and to analyse what was done. If you say "accept things that are quite obviously not addressing the needs", you have to substantiate it.