Solid rocket motors are so much simpler than liquid fueled. Why isn't e.g. SpaceX making a Falcon 9 equivalent with solid fuel only, and rescuing the segment cases with parachutes like the shuttle SRBs?
The reason is refurbishment cost and turnaround:
For a liquid booster, you can more or less pump more fuel and you are ready to go.
With a SRB, it isn't so easy: You need to dismantle everything, check it and build it again.
Space Shuttle's SRB refurbishment has been quoted as a marginal improvement over building them again from scratch, altho I couldn't find a reference on it.
There are a few reasons that re-usable solid rocket boosters are not being built. The biggest reason is that people don't want solid rockets in the first place. Solid rockets can be stored loaded long term so they are useful if you need a rocket to be instantly ready to go, for instance military purposes, but they have a much lower specific impulse than liquid fuel rockets, so a lot less power. The military doesn't have a use case for recovering rockets, and the space industry isn't looking to solid boosters for launch, so no use case no development.
Solid boosters are also harder and more expensive to recover because once they are lit then cannot be throttled or stopped and re-lit. You can't have a solid rocket land itself like some of the liquid fuel rockets are, once they cut off they are going to be downrange and you'll have to go find them, haul them out of the water (assuming they wet land), then bring them back and refurb them. Refurbing them means you have to take them apart, clean all the spent propellant out (a nasty and toxic job), and put them back together again. Once the economics of it are worked out the cost of recovery often doesn't make it worth the trouble - look at the shuttle's SRBs as an example. It's possible that technology could be developed to allow the fine control of solid boosters, making them more easily recovered, and the maybe they will look at the possibility.
Small reusable/reloadable solid motors (from model rocket size up to about 40,000 lb-sec) are commodity items available from multiple manufacturers. One of the biggest users of the reloadable case system turns out to be emergency parachute deployment systems for ultralights and light aircraft.