It's tough to tell without specific names, but I imagine a lot of particularly old* VSTs were written to use naive sawtooths rather than perfect band-limited ones, which would have terrible aliasing at 44.1 khz. Oversampling those would help a lot!
* Some people are still making this mistake, despite information on the (many) ways to do it the right way being widely and freely available!
* Some people are still making this mistake, despite information on the (many) ways to do it the right way being widely and freely available!