That’s because BW is an early RTS, made before there was such a thing as pro play outside of tight knit communities.
Unlike SC2 units in BW weren’t designed with the sole purpose of efficiently killing workers, but some excelled at it better than others. Zerglings, Vultures, Reavers, Mutas, etc. were all good at killing workers but they served other roles as main army units and 2-3 of them couldn’t kill a dozen workers in 5 seconds (except for the reaver, which was a heavy tech commitment).
Furthermore, if you committed to a harass heavy style you were very open to a powerful counterattack from main army units unless you killed workers early or immediately transitioned after the first wave of units (but there would still be a window for counterattack). This still happens in SC2, but not to the degree that it did in BW. If you went 2 port wraith and didn’t cripple a Zerg’s economy, you would get stomped by hydras.
BW also didn’t have runaway macro mechanics, so replacing workers and building deathballs took a lot longer, leading to a lot more skirmish/push/timing based gameplay as both workers and units were relatively more valuable.
Personally I agree with the OP, harass shouldn’t be such a dominating strategy in every game, or it should be less forgiving for the harasser. Watching combat units slaughter units that don’t fight back until the final deathball vs deathball fight is incredibly boring.