It's certainly a little weird, considering they can pinpoint 95 MPH pitches, but there are a few reasons. Throwing to second, they have to spin around quickly, locate their target, and find their footing and proper release. They aren't going to practice that nearly as much as an infielder practices throwing, if at all. And when you think about it, infielders don't typically have to do a 180 like that.
In addition, going from repeatedly throwing off a mound to instantly off of the grass can sometimes be a difficult transition with no warmup throw. On top of that, they are often partially on the mound, not even on flat grass.
When I pitched, I tried to deliberately set myself (if I had time) when throwing to a base and then really whip it to make it feel as much like a pitch as possible.