Giving up is just the wrong. It's the lazy devs and lazy steam. If you don't define an age rating as a developer you can't sell a game in Germany. It's just a simple form on steam you have to fill out as a developer to get an age rating on steam.
Lazy steam because for 18+ games they just need to implement a working age verification like the German free postident where the game seller (in this case it would be steam) sends you an email with a code and you walk with the email and your ID to your local post office and they check if the age and name on the ID is the same it is in the mail. That is a one time test and you are verified for all 18+ games from that site. I did it for another site and it takes less than a minute. They just do not want a 10 year old kid playing Rust or Tarkov or other games for mature persons. But steam thought it is easier to region-block all 18+ games and lose profit in that region....
You are writing this from the user perspective. From the perspective of Steam, they would have to implement special code to distinguish between adults and verified adults, and they would have to implement a special verification system that only exists in Germany. And that might not even be enough. There is no guarantee that the person actually using a PC or Steam Deck is the account owner themselves rather than their child or younger sibling. What they have chosen to do instead is forgo a relatively small amount of profit and save all this effort and risk, while indirectly exerting pressure on the German government to be less strict -- or push for a uniform solution for the entire European Union, which it would actually make sense to implement. Meanwhile, German Steam users who really want to buy adult only games can find someone in another European country who buys it for them, for example from Austria.
As to 'lazy' developers: The presence of some old games on Steam is basically just passive income. The developer is not doing anything with the game any more other than (if required) pay tax on the revenue they are getting from Steam. Maybe they do not even have access to their developer account any more. Or maybe all emails from Steam get deleted automatically, or go to an email account they never log in to. So they either don't learn about the problem, or can't fix it without too much effort.
The underlying problem here is unreasonable demands from the German government. Requiring the developer of a normal classical roguelike or even solitaire program to fill in a special form just for the German market is insane. This would be problematic even for actual adult games, but when it's so obviously not even a borderline case it's just a malicious trade barrier. And requiring extra strict age verification of a kind that only exists in a single country is also not appropriate.
By the way, even from the user perspective it's not that easy. My recent experience with Postident has been that if you want it to be relatively easy you absolutely need a smartphone (I lived most of the recent decade without one, for reasons of psychological health), and even then you need to install an app to get it done in approximately 5 minutes. Of course this also means that you can assume that from then on some Internet-facing server, likely to be hacked at some point, is going to store data connecting your name, address, date of birth, email address and mobile phone number. Enough to steal your identity once that server is hacked, which seems inevitable.