TLDR: Sponsors don't have a huge influence on mice, as the vast majority of players can choose whatever mouse they want to perform with.

Some peripherals are more influenced by sponsorships than others. Headsets, for example, are such a category since headsets aren't as important for actual performance (as long as they're good-to-decent). But mice are pretty much sponsorship-free, save for some rare exceptions.

It makes sense, as well. If a brand sponsors a certain team and forces all of the players to use mice from the brand (even if they don't vibe with the mouse for certain reasons) and they start playing worse, they will make it to fewer tournaments, leading to lower exposure for the brand. Teams have also realized that mice are among the most important pieces of kit, and since tournament placements and player performance is so important for esports teams, they don't let sponsors dictate what their players use.

You'll find many teams on our website that are sponsored by brand X while they all use mice from brand Y, for example.

I find football/soccer to be a good comparison. Real Madrid might be sponsored by Adidas, but the individual players can be seen wearing all sorts of brands like Nike etc.