Because carriers are a massive engineering project whose benefits to the economy can be spread between a number of states and congressional districts allowing them to get funding approval. Smaller ships would require much less. A carrier group is basically a floating military base that generates a lot of economic activity. The US military is first and foremost a welfare program for our weapons manufacturers; it’s why we spent the last 20 years throwing teenagers into a meat grinder with no real mission objectives other than to destroy a lot of equipment so it could be replaced.

I agree with you that carriers are a relic of the 20th century. Manned aircraft probably are too, which negates the need for carriers (which I expect we will see in the next major air war).