We often used to get into their floor time pen when the guinea pigs were out of their cage.
My kids would often sit or lay in there reading or doing homework, and the piggies would wander over and climb into their laps or nibble the pages of their book.
It was a very fun and safe experience for everyone, and the piggies were always free to walk off and do their own thing too.
I think in a small space on a desk they might feel scared and would also be at risk of jumping and falling.