Why do bike tires get hot?

Depends on what you mean. If left in the sun they will get hot from soaking up the sun's rays, just like anything else left in the glare. If you lock the wheel up and skid you'll get a hot spot just where it's slid on the asphalt. If you've got a rim braked wheel and you've just done a serious descent (think Rockies, Alps....) then some heat will transfer from the rim to the tire. In theory you can get the tires to heat up from the deformation they see as the bike roll along, but I can't think of any realistic scenario where that'd qualify as "hot".