On blitz, players don't have time to analyse positions deeply and prevent traps from people, and on high rates there's a good possibity that your opponent knows more traps than you, because it's their main opening, so it's only natural that you either try to dominate a certain ammount of openings, or you can either try your luck in middle game for which you feel more confortable while your opponent won't have the expected game he was working for, a try an "alternative" or less used opening.
About moves like b5, g5? Well, to be honest they're far from bad moves, specially coming from high rated that probably know exactly what they're doing, they're arguably even "master moves". Specially b4, a3/a4 pawns, along with Rb1, Bb2, and Nf3, and you're half way through the possibility of pawn breaking queen side (or king side respectivelly) and have very energectic attacks that can demolish oppositions.
There's also the secondary aspect that b4 and g4 can work as a pseudo gambit that allows c3, d4 in grob attack as well as pressuring opponent knight and c pawn.
Beyond avoiding tactical pitfalls, is it wise to play classical openings in Blitz?
Often I see high-level competitors using very silly moves in Blitz, but getting away with it because there (generally) is not enough time to properly analyze the move and exploit the weakness.
Moves such as (black b5 g5, white b4 g4) in the first ~10 moves is generally considered a mistake if its unsupported, but it seems to be common in high level blitz.