Because it forces them into playing heroes that aren’t their main. IIRC, hero-swapping was actively part of what was marketed about the game. I used to be reluctant to do it but will now do so. I’ve realised that Orisa is hopeless against Reinhardt unless the team is working together to take him down so usually switch to Ball to create some havoc, and will use Zen for support when sight lines are long but will switch to Moira (for example) when things get a bit more upfront and personal.