Except we didn't, because we took absurdly high-end engines (RS-25s) that were designed for reuse and refurbishment and now we drop them in the ocean after every launch.
RS-25s were the three main engines. They were very expensive, designed for reuse and were recovered with the rest of the orbiter they were bolted on to. Then refurbished with a much greater amount of effort and money than initially expected, and eventually reused on a future mission..
The SLS uses two similar boosters and FOUR of that same(-ish) engine design. The boosters can be recovered just like before. But the first stage doesn't fly itself back to Cape Canaveral after 2 weeks like the Shuttle did. So those four very expensive "reusable" engines are chucked into the ocean never to be seen again.
The shuttle's SRB's were fished out, refurbed, refueled, and reflown.
The SLS's SRB's are left to sink to the bottom.
https://space.stackexchange.com/a/45894
Basically the SRB had multiple modules and some were more reusable than others, so some got recovered and refurbished a lot more.
>it would cost $23 million to refurbish a used SRB and $12~70 million to refuel it.
A unconfirmed sources, that worked at NASA claim that Thikol employee explained to him. That reuse cost 3 time more, than a expendable SRB https://forum.nasaspaceflight.com/index.php?topic=51959.0
Building Saturn V's at low volume under the standard cost-plus arrangements that NASA uses with Boeing et al would result in steadily increasing costs.