How long is garlic butter safe, and why is it not a botulism risk like garlic in oil?
There's no reason to believe it's safer.

Garlic in oil is "unsafe" by FDA standards.  Which means that roughly one in 100,000 bottles of homemade garlic oil kills someone.  Before reading about the botulism risk, my friends and I used to make garlic oil at home and hand it out; I'd say we distributed probably 100 bottles, some of which stayed on the shelf for years before being used.  In that time, nobody got sick from it (most bottles went to friends, so we'd have heard).

So the fact that your garlic butter hasn't killed anyone yet just means that you're playing the odds.  Chances are, unless you get really sloppy, you could go on making garlic butter for the rest of your life and never get botulism poisoning.  But not everyone is comfortable with that risk.

EDITED PER BELOW:  You can improve your odds of avoiding botulism by straining the oil/butter through cheesecloth (to eliminate solids which would hide spores), and heat-treating it to 160F or more for 45 minutes.  This will not eliminate all risk of botulism, but will improve your odds.