It bothers me too, there are many valid 3-letter words or elements I want to search for.

I tried to change it once, but then I noticed that search queries get sent to some kind of specialized search database. I don't know how it works, or what kind of software even builds the search indexes, so I stopped trying.
My guess is that it's configured somewhere to require at least 4 characters before searching, to reduce load.

Anyway, I implemented a 4 character limit inside TPT because otherwise it just returns the top votes saves instead of anything relevant to your search.