Many games re-release as "complete" editions with updates and sometimes DLC. I don't know if those always find their way on to the disc proper, but unless they're relabeling and re-boxing dead stock, I have to imagine "stamp it with some extra bytes" is more economical than "make all-new discs put in all-new boxes but it's just the base, non-updated game AND we have to keep the servers running."

I just checked around, and according to Doesitplay?:

- over 1/3 (34%) of PS5 games on disc from their sampled games are either too buggy or missing required/advertised content without downloading something from a server before first play. 13-18% are unplayable without an initial download.

- 40% of games on Xbox One/Series X are too buggy/missing required or advertised content, and 10-11% are unplayable without an initial download.

Some of these games may even have hardcoded download requirements.

https://www.doesitplay.org/methodology