Concerning PBKFD2, 2048 iterations was designed to slow up brute-force attack. But 15years after this design, 2048 iterations is quite weak. Do you think we should make more iterations ?
BIP39 was only proposed in 2013 and the 2048 iterations was still being debated in 2014, so this is only 7 years old, not 15. 2048 was weak then and it is weak now. As a result, the security of seed phrases against brute forcing comes largely from the entropy of the seed phrase.
There are certainly good technical reasons to increase the iteration count, but there are practical reasons not to. BIP39 is now so ubiquitous, that if you started introducing new iteration counts it was cause havoc. There are still plenty of people who think they have lost their coins because they don't know the difference between recovering a legacy wallet and a segwit wallet. Electrum seed phrases cause frequent problems as people do not realize they are different to BIP39 seed phrases. Having the same seed phrase able to recover multiple different wallets of the same type due to different iteration counts will only compound this even more.