Some lodges have something going on several times a week. Pancake breakfasts, stated meetings, degrees, social clubs, ritual practice, etc. Others have one stated meeting per month and go dark for a few months out of the year. If you lived in an area where the lodges only got together 9 times a YEAR, that might not be enough for you. But if you joined several different ones, together they might keep you as active as you'd like.
Or, I've got friends in Lodge A and friends in Lodge B and I enjoy the company of both, I think I'll go to both. Or, as Brother MRichard pointed out, Lodge A is social and Lodge B focuses on philanthropy and Lodge C hosts a great discussion on Masonic topics. I like all three of those aspects but can't find them all in any one lodge.
There's a number of different reasons.