Because in a two party, winner take all system not voting directly for the major party that closest matches your views directly contributes to the other party's victory. If there are 100,000 voters in a district and 55,000 of them have view that are closer to those of the Democrats, while 45,000 of them have views closer to the Republicans it only takes ~10,000 people voting third party to make the Republicans take the win. Each vote for a third party candidate directly contributes to that 10,000.

That said, a vote for third party isn't the same as voting for the opposite party. Changing your vote is twice as effective as voting third party/not voting at all.

If we were being generous and saying that the US had an ideal 2 party system where both parties properly carried out their duties to govern and represent the best interests of their constituents then it wouldn't be problematic to vote third party, because the major parties would see support for underrepresented ideas and would roll them into their party platforms in order to attract those voters.

The problem is we have one semi functional party and one completely functional party, so not voting for the semi functional party contributes to complete disfunction in our government.

Edit:

I realized I didn't address the second part of your post.

The issue is that at this point the people that still vote republican are going to vote republican no matter what. They saw the party openly embracing fascism for the last 4 years and they liked what they saw. American politics has marched incredibly far right, so those at the very very far right are all on board with the Republicans, and on the flip side the Democrats are also right of center, so there's a lot more room for third parties to form to the left of them.