They are enormous fun. That cannot be a bad thing.
They are also helpful with pedalling technique.
I built mine for fun, but my riding improved at the grand old age of 40+.
A year or so after putting it together, I found I was regularly riding at 110 rpm, something I'd never done before.
Mine is made of an old 531 steel frame and odd bits and bobs. I do not understand the desire to buy a new one, but it takes all sorts.
Up to a point, I think they are surfing on a fashion wave, as early MTBs did in 1990-ish.
That need not be a bad thing.
The only reason to get one is because you want one. If my garage were on fire, the fixed-wheel fixie fixopholous might be the first thing I'd rescue.
It might also be my Pace HT MTB or my lovely Italian alloy road bike.... Blimey... I might end up getting quite hot.