The value of stock speculation is that it gives the original owner a way to raise capital for a company that could make money later but doesn't yet. It's how founders and growing businesses raise money. And the prospect of speculators being able to sell to future speculators makes the original speculators willing to pay more to the business itself.

A speculator in these markets also has the incentive to preserve the art or stamp and protect it from damage. Which also applies to stocks, because the speculator has the incentive to vote their shares in a way that maximizes the value of the company.

None of that is happening with domain names. The squatter didn't create the name, no one did and no one needs to be compensated for that. If no one "maintains" the name it doesn't degrade in any way, it just goes unregistered until someone wants to actually use it.