The legal profession self-selects for people who think this way, for a number of various reasons.
1. If you disagree with the base assumptions of copyright, you are going to misunderstand the law and fail your LSAT/bar exam/etc. You will be blinded by "it's just to keep big companies afloat" to notice the actual rules of things like fair use.
2. If you get a law license but disagree with the base assumptions of the law, you are going to be at a financial disadvantage by refusing to represent clients whose politics disagrees with yours.[0]
3. If you choose to represent copyright defendants, you can only do so to the extent that there is a legal argument that they can make. It is illegal for a lawyer to aid a client in the commission of a crime... and if your legal career is based around the idea that something shouldn't be a crime, then you will need to hold your tongue and pick your battles a lot.
4. If you somehow jump through those hoops and competently represent clients... you probably have sacrificed on your "abolish copyright" ideology a fair bit.
[0] Remember that at this point you are probably deep in student debt - your legal career is predicated on you paying back your loans by charging your clients lots of money.