The solid rocket booster casings were dropped into the ocean and (usually) recovered.

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.

They've given up on refurbishing & refueling the SRB casings for the SLS.

Simply having to maintain one or more ships (continuous expense, year round, year after year), to fish those tubes out of the ocean (once every few years) almost certainly ate up any cost savings they could possibly get from refurbing the tubes.

And this lazy, reductive line of thinking is how they got to $4B/launch.

Lmao, do you have any idea how much ships cost? The spent SRBs being sunk are the least of SLS's problems. SRB shell refurbishment had dubious economic sense when Shuttle was flying several times a year, but for something that will fly as few times as SLS it would be an absolute farce.

And yet, the SLS does.

The shuttle did not drop RS-25's in the ocean. The SLS does.

The shuttle's SRB's were fished out, refurbed, refueled, and reflown.

The SLS's SRB's are left to sink to the bottom.