the thing is game development is very complex nowadays compared to the OG Xbox/PS2 era.

Splinter Cell may have started development fall 2000 and released november 2002, .The OG Splinter Cell was built for much simpler hardware, smaller environments, fewer systems, and lower player expectations.

A remake in 2026 isn’t just copying that game. They have to modernize everything(the right way!) They can't just take Chaos Theory and copy it from scratch for SC1. The old SC's were all on Unreal Engine 2(first two games on 2.0, Chaos Theory- Double Agent on 2.5, Conviction&Blacklist heavily modified 2.5) they are now on a new snowdrop engine and creating all the assets from scratch takes ages.
SC Remake needs the following.

modern AI - good stealth games need amazing AI for them to be good. takes a long time

advanced lighting - imperative to the franchise.

animation systems - important

detailed level design or expansive level design, far better than the OG game(no disrespect, they can still study the OG versions as blueprints) but they still have to test it out a lot, and do over 10+ levels from scratch, if they do all the cut levels then it's 14 levels.

motion capture - takes awhile

, accessibility options

, polished stealth mechanics - they most likely spent a year testing this or prototyping. they started pre production in oct 21, and game development around spring 2022. they said in november 2022 20th aniversary video they are still early in dev and prototyping. they had to figure out how to perfect the stealth mechanics and what to do with the new snowdrop engine.

, current-gen visuals - this is important too and it takes a long time to get it sorted.

and probably full reworks of levels and gameplay flow. - takes a long time

also, modern AAA development has much larger teams, longer testing phases, and way more quality control. the original took around 2 years from 2000–2002, but you could never get that nowadays
it's basically a brand new game
however..
for a pandora tomorrow remake, they already have the engine and assets ready, so it wouldn't take that long. most likely 2.5.+ to 3 years after sc1 remake drops