But "freestyle" in competition ist freestyle. You can chose any style you want, but crawling is the fastest so everyone crawls.
The problem with swimming under water is, that you have to make a definition where swimming ends end where freediving begins. You can change the definition and say: OK, in freestyle you are all allowed to do the whole line under water.
Then every swimmer would train to swim under water because it's faster. They are not more free in choosing their stroke then, they only adapt their technique to the new rules. People who swim races normally do that to compete with others, not to see if they they are slower or faster coming to the finish as the slowest of all others. To do that I need no competitions.