How does heat turn things into rubber? The meat I reheat was already cooked? Or am I always too gentle to experience that? I usually do not heat up to boiling hot since I want to eat it and not have to let it cool down first.

Again, your generalization does not make sense. 50 % of a 1000 Watts microwave is different to one with 600 Watts. Heating up a bit of leftover is different to something for multiple persons. Etc. etc.