In part, because even simple annuities pay poorly. According to immediateannuities.com a 5 year period certain annuity will pay $1,720/month for 60 months with a $100,000 deposit. That is about 1.25%/year and you have your money tied up for 5 years.
OTOH, an online savings account pays close to or the same rate of return, is FDIC insured and you can get your money anytime that you want.
So if you had a choice between tying up $100,000 with an annuity company and getting $1,720/month for 5 years or getting the same return but having access to what is left of your $100,000 at any time you wish, what would you do?
If you buy payments for life, the same low rate of return is embedded in the monthly payments that you get, it is just harder to see.