Why won't an oven do? A couple of reasons that I can think of:
1) I cook in my oven, that means there's a lot of fat and soot just waiting to drop into anything that I put in there.
2) I clean my oven with some pretty harsh chemicals, the residue might be harmless to eat but to inject?
3) The air in my oven moves around including pulling air in from outside the oven (although in small amounts) that air can't be considered sterile.
4) A dry heat sterilizer ran me $150 on Ebay and I've read studies showing that if your equipment can handle it dry heat is more effective than chemical or steam/pressure (autoclave) sterilization.