Because lets face it.. it would actually be really easy to do, and you could create a lot of different variants.
as for the rest of this suggestion, if you don't understand by the Title here it is.
Make Golems with more or less health based on different blocks used in their construction, Diamond would have a lot of health and do more damage than iron, gold would be fancy looking but little more than decoration, Stone would be like construct-able Wolves in terms of damage and durability, and Netherrack would be immune to fire but otherwise quite weak.
oh Maybe have Netherrack make a altered version of the snow Golems that shoots fireballs. that makes sense right? i just wouldn't want it in my house...
i say this would be a easy mod to make because creating slightly altered copys of mobs isnt that hard and with some skill in making the textures would be something that could be fired off in a weekend.
Lets face it.. i am not the only one who considered this idea when he/she learned about the new iron Golems. I might not even be the first to suggest it on these forums =P
- Glowstone golem: Drops Glowstone dust and Netherrack
- Soul golem: Drops Soulsand and Netherrack
- Blaze statue: Special golem, is a statue until it sees you. It guards the Nether fortresses. Drops Blaze powder and Netherbrick.