after pistol round

why do so many teams buy 3 rifles and 2 pistols? who first made it? wouldn't buying 5 spectre be better?

It was part of NAVI (or then FPX) main tactic when they won masters last year. It basically means the 2 pistol players can die on round 2 and they don't lose value ( if guns die then the pistol players pick them up). Then they have full guns to fight the bonus round rather than having tons of SMGs Vs rifles. Not many other teams do it as well as they do but people have copied it, fnatic do a similar thing with marshalls as juiced ghosts