There are some out there, but not at the commercial/industrial level. Like you said, higher risk along with added shipping cost keeps them out of the mainstream. Also the need for lightweight UPS systems for servers is pretty low. One bad thing if a lead battery is involved in a fire is the spreading of lead contamination, that's not cheap to clean up, but it's low probability.
Not the same battery chemistry, so much reduced (if nearly zero) fire risk from a UPS.