FSA money is "use it or lose it", unlike HSA. My understanding was that anything "lost" went to the plan administrator, so it might be a bonus to using one. (Although, in the specific instances I heard of, it didn't come close to making up for the administrative cost.)
unless you have kids in daycare/prescription meds a complete crapshoot for the employee
This is likely a much higher percentage of the employee population than you're thinking. And, strictly speaking, any chronic health condition would fit, not just prescription meds.
Also, offering many options is going to be more costly than just offering one. You won't get the strength in numbers with any one plan that would help lower costs, and it's going to be more work to administer. HDHPs are great for some people, maybe even most, but those with chronic health conditions are likely going to find a traditional health plan cheaper overall, and there are people who have had traditional plans for decades and are resistant to change.
I would bet that someone at your company has done a cost comparison on offering multiple plans, offering just traditional, and offering just a HDHP with some HSA contribution that would make employees whole in terms of total OOP health expenses, and determined that traditional is the cheapest.