Given the dangers of the massive heat generated during re-entry, why can't spacecraft simply slow down using thrusters (or whatever) and just come in gently?
Because propellant is extremely costly in orbit (as of now). As long as you have to bring up every gram from earths gravity well, you want your tanks to be as empty as possible before re-entry. Aerobraking is currently the only way to get rid of speed without relying on costly propellant.
That being said, things might change if propellant would already exist in-orbit and was relatively cheap to acquire. In that case it might be more sensible to re-fuel (re-propell?) before descent.