i will repeat myself yet again: it is not easy to understand how natural selection works … even the researcher below is quite surprised (these guys are always surprised):
Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been subjected to strong selective constraints due to the crucial role that they play in species survival. It is therefore quite surprising that the ability to develop functional teeth has subsequently been lost several times, independently, in various lineages.
full article:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2736120/
and here are some other researchers who seem to be surprised too:
Key evolutionary innovation lost 1000 times! The ability of flight lost repeatedly in the vast majority of insect orders