Because it trivialises the weight of the original choice.
E.g. consider marriage. If you knew you could only divorce for a massive cost would it change the way you pick a partner Vs. If divorce was instant, for free and a new partner selection was guaranteed? Would you also invest the same amount of time in either case?
Answer honestly
That's a pretty bad analogy, in fact, if you wanted to use such an example, not having a respec option would be more akin to not having the option to divorce your partner. Divorces are pretty common because people make mistakes and sometimes things turn out different than you thought they would, so if you're unhappy with your marriage, you get a divorce. It gets ridiculous when you put it like this, the people arguing against respec are people that are happily married, that don't want other couples to be able to divorce even if they're unhappy, and that apparently couldn't stop themselves from going for a divorce even when they claim they are happy in their marriage, just because that option exists.
Yikes.
If you don't want to respec your character, just don't do it, it is not a mechanic the game pushes you to interact with, if you can't prevent yourself from doing something you don't want to, that's a you problem.
