some of this companies already have a huge followership, that's a huge number of loyal customer base and they feel they don't need to have a sign up bonus in place to meet their daily target, alot of gambling sites I've signed up too didn't offer any sign up bonus.
And another reason is some of the gambling sites feel that putting sign up bonus in place encourages people to have multiple accounts which usually ends up being dormant accounts, and some users end up using those accounts for some kind of illegal activity which puts the business at risk.
Long time ago, I had a friend who will sign up on a gambling site because of the bonus, he will deposit the amount required for him to be able to use the sign up bonus, he would bet with the sign up bonus, after some time, if he doesn't win, he would withdraw all his money and create another account on the same gambling site to enable collect another sign up bonus.... I guess this is one of the things most casinos that do not offer sign up bonuses are avoiding.
That is why a signup bonus can come with terms and conditions. That is to prevent abuse from greedy people but sometimes the terms and conditions to complete the bonus are a pain in the ass that even a legit account can't achieve. No wonder why there's always a complaint about these so-called bonuses.
Casinos should try to be realistic too sometimes. For the gamblers, if you think that bonuses are hard to get then you should not focus on getting it because you are only risking you money at the end. Gambling is supposed to be built fun and not to pressure anyone else. Better if should focus playing on the main game.
It is because of this that it is important to read the terms of service of each casino before we deposit any money, because as you state there are some casinos in which the wager requirements are extremely harsh and if you are not interested in the games which count towards those wager requirements then you are never going to fulfill them.
And if you decided to try then you will have a bad time paying at the casino as now you are just playing those games not because you want but just to clear the wager requirements, it is because of this that when I see wager requirements like that I reconsider playing at that casino or ask the customer service to not give me the sign up bonus to avoid that scenario.