Why isn't "I'm glad to hearing from you" correct?
I don't think grammar books say that you always use gerund after any 'to' :) . One thing is the phrasal verb 'to look forward to' which requires noun/gerund; the other - the adjective 'glad' which requires 'about', 'of', 'for', or an infinitive (hence 'to').
See: http://www.freecollocation.com/search?word=glad
So: 'glad to hearing' is incorrect because
- if 'to' is a preposition, then it is a wrong one - 'glad about hearing' is correct
- if 'to' is a part of an infinitive, it should be 'glad to hear'.