They could have allowed it and they didn't. They and you are assholes.
Yes, they could have. But business is rarely done along the "it is easier to ask forgiveness later than seek permission now" principle.
You don't build a Mickey Mouse watch first, advertise the heck out of it, then come to Disney expecting to get a license agreement hammered out after spending all that money. In fact, Disney is more likely to screw you over knowing you spent all that money in the first place,
What you do is ask Valve first to license the IP - you have this brilliant idea and would like to acquire the license to produce it. Chances are, Valve will make a commercial license agreement and we'd have our sides.
It doesn't mean dbrand can't make their own sides - unlike what Sony did to their PS5 replacement plates.
In fact, the law generally gives those who try to ask forgiveness a huge disadvantage - sometimes allowing for treble (triple) damages. Or even meaning a forced license agreement - that is, the other party gets to make all the commercial terms and you're forced to agree to them (this can happen on things like stolen land - where your neighbor builds on your land by accident. You have many options, including selling them that land - at your price, Your neighbor basically is forced to buy your land at your price, or vacate.)
And we don't know. Maybe they did try to reach out to Valve and after Valve saw they put so much time and effort into it, they demanded a huge licensing fee. Remember - you license first then build and if it goes well, everyone benefits. You build first, then try to license it and now the IP owner sees you're going to make a lot of money, and they can demand a lot of money in return.
Maybe Valve would normally demand say, 20% of the retail price for licensing. Now they see it's successful, they could demand 50% because they know you're desperate for the license and have the marketing information (pre-orders) to justify it.
Commercial licenses are pure business transactions. dbrand's big mistake was not licensing it before making it public. Valve likely saw the publicity and offered them a "take it or leave it" licensing deal that basically was "you saw the demand, we saw the demand, now pay up". And they ran the numbers and likely Valve's fees would've meant they made almost no money on the things.
Remember, asking forgiveness in business usually means getting screwed over. Even the least crafty businessman in the world knows if you're desperate for a deal, there's more profit to be made. dbrand went from "licensing potential" to "desperate to license" which means easy money.
And given what we know, this is exactly what happened - they showed it off first, got a ton of pre-orders, then tried to ask Valve for a license. Valve saw what happened, decided to go for easy money and offered a license at rates above normal because they saw the public numbers and would be an idiot not to take advantage of a golden goose that landed on your lap. dbrand refused to accept the terms, Valve refused to negotiate, knowing dbrand was in the weaker position overall (they were desperate, and they had a ticking clock).
That's just business. They probably should've asked first, not ask later. And no one will convince me otherwise that they'd do the opposite - you'd leave your job the instant someone offered you a significant amount of money or better things like WFH or other things - because it's all about money.