Counter Strike has been the same game for 23 years, basically every new game is an "update". Porting the game to Source 2 is the single biggest thing that has happened to Counter Strike since 2004 when they moved from GoldSrc, so might as well brand it as a new game.

I have 4k hours in CSGO alone. Probably 10k hours in the whole series all together. The changes they made are far more significant than an update.

Just update a current game and put a 2 on it?

It was a "ground-up" rebuild on Source 2, so while it carries forward all the CSGO content and aims to "play the same" in terms of movement and gunplay (with the exception of improvements like subtick actions), I'd say it's way more of an actual "2." New engine with all kinds of fancy lighting and other improvements, new assets (including weapon and character models, some of which were still originally in the 2013 CSGO launch), remakes and retouches of maps, vastly improved map-making tools, some nifty accessibility features (your walking sounds appearing on the radar) and quality of life features (selling back misbought items, or the picture-in-picture grenade throw practice camera), and some huge balance changes (games are now shorter, players now need to more strategically choose their weapons, smoke grenades are voxel-based and can be cleared out with gunfire and grenades, skyboxes are now open for grenade tosses, etc.)

It looks the same but with some lighting changes on the surface, but it's actually huge.

Despite some minor changes the game has played fundamentally the same since 1999.

Edit: also CSGO came out 11 years ago, compared to the 6 years between overwatch 1 and 2.

Just update a current game and put a 2 on it?

This is how the software engineering industry works. It's actually games that are the outlier here, with their big releases each time a sequel is made.

More visibility would definitely help here though, odd that they don't put a banner on the store given how big of a release this is for many players.