Most are aluminum with a steel sleeve. Is there any reason they can't be made in solid steel?

No

Why carry the added weight

If you make them out of steel i would put some grease on the spring eye bolt.They could be a pia to remove if rust sets in.

I'm with you on the weight, MrP, this was a conversation with a guy at work. He's doing a Brand X off roader, and from all appearances seems to be on a mission to make this thing as heavy as possible.
I had never seen a set in solid steel, so I told him I'd ask.

Solid aluminum with a steel sleeve is all that have been out there so far.

My Calvert springs came w/an aluminum bushing but didn't notice a sleeve..............

Mine have a steel sleeve that the bolt goes through and the belcranks pivot on.

For an off road rig I'd probably do yellow brass instead of steel.

it has to have a steel sleeve and you must have missed it or else it will eat itself out of housing and home.

Maybe brain fade cos I don't remember sleeves being there and it`s been a while since I had the leafs completely out.......

and if it doesn't do that, once galvanic corrosion sets in, the bolt will weld itself to the bushing, making a real treat to remove. i have learned many new words over the years dealing with this issue. it's not so or too bad when there is more than .020-.030 clearance between the bolt shank and the bushing hole, but that just accelerates the hole wear. when you get the slop out of the bushing [as it should be to be effective], that allows the two dissimilar metals to corrode together locking them solid. then the fun begins. and even using a sleeve, anti-seize or high pressure grease is a must for best performance.