No reason at all.
I buy my meat from the normal shop, from butchers, from online shops, etc. As long as it's meat that's meant for humans it's fine. Also offcuts that would not normally be sold as human food are fine, although I don't feed that myself.
I've heard of people who collect roadkill and feed that as well - wouldn't recommend that though, as you never know if they'd ingested any poisons etc.
You should make sure you feed whole pieces of meat and bone as well as the mince though, as feeding mince only would not be sufficient and their teeth would suffer.
The ratios you need are roughly:
90% meat (including heart)
10% bone
10% organ meat (liver, kidneys, etc)