In other words, these are comparatively fewer of these guys than the US president, millions of times fewer than elite navy seal units or army generals or joint fight strikers, fewer than aircraft carriers that cost billions of dollars each and can house a hundred aircraft each worth hundreds of millions of dollars, and all military more significant, each and every one of them, after all you as one of the spectres is about to same the freaking universe.
Yet the player has to buy freaking omni-gel with scarce credits. Meanwhile you're flying across the universe in one of the most technologically advanced, trillion dollar spaceships, saving that very universe. In fact you employ said spaceship to go on missions to
That's like a carrier with joint strike fighters and nuclear bombs and the general of the navy, has to freaking sail across the oceans in search for somalian pirates that they can arrest, so they can make some money on their bounty, so they can buy some freaking bandages and rations, while simultaneously being on a mission to save the planet from impending doom lol.
There's not a lot of games that get this right. Tons of games get it right because the character isn't as important. Like in Pokemon, you're just a kid from a little village with ambition. But if you're the most important character in the universe, please don't have him pay for his bandages and ammo. And even in a convoluted 'everyone thinks I'm the bad guy, so nobody funds me, but I'm actually trying to save the universe', if you have universe-saving skills, just rob a freaking bank or something, it can't be that hard in comparison.
Witchers were relicts of different times and had to compete on price or pivot to assasin/bodyguard industry :)
In games for gameplay reason they made monsters very common again (explaining it away by wars and plague decivilizing big regions again) - so it make sense for witchers to be more wealthy again.
The world of the Witcher 3 is a lot more monster-riddled than what's presented in the books, with a board full of witcher contracts at every little settlement. It wouldn't make sense for witchers to be poor in the context of the game, especially in a world where witchers are a dying breed and monsters are on the rise again.
Remember that entire villages pooled their money together to pay for a witcher's fees. Also, monsters are not as frequent as they were a long time ago, so contracts are rarer and lower in payout, especially during a horrific war.
And despite what service they do to people, they are viewed as freaks (since they are mutants) and are marginalized by the society. So earning a living is not an easy thing for a witcher.
Graphical illustration of that concept: https://www.artstation.com/artwork/merely-tolerated