Several reasons:

Easier to animate, yes, but also easier to streamline when everything was done by hand;

In black and white, some of the earliest animations showed black characters (like Mickey and Pete) without gloves and the hands blended in with the bodies during movement, so the white gloves helped break it up visually;

In Mickey’s case, they didn’t want to animate mouse hands (lol);

A direct connection to vaudeville, where performers traditionally wore gloves — apparently many early animators were also vaudeville performers.