Some yarn stores offer finishing services, where you can pay them to sew your seams for you if you truly truly hate it.
But I agree, a sewing machine isn't really the best way to sew those handknit sweaters together.
I've had good luck with a different technique that kind of tricks me into seaming the side seams.
The right sides are facing each other, the needle goes through both layers in the top row of stitches, and I pick up stitches all along the edge, then I knit the row and bind off all of the stitches on the way back.
It's SUCH a cheat that the knitting fairy will probably come down and slap me one day, but it's worked great on a half dozen tops so far, and it's so much quicker than mattress stitch for me!!!