Bonuses are actually honey pots. They are not there to be given out for free. The terms and conditions on those bonuses are quite hard to reach and in the end you will actually just lose everything before you even get the bonus. That is why bonuses on gambling sites feel scammy, because it is almost impossible to get the bonus anyway, they just make you want to bet more and more without realizing that the fact is that it is next to impossible to do the roll over requirement.
Absolutely true for the most parts. Especially on those casinos that offer slots (thanks to the extremely high edges on slots) - this is where you find those impossible to believe bonuses of 200% to 500% even. All the rollover requirements end up putting you in tough -EV positions that really do require you to be extremely lucky, or play really long streaks of microbetting.
I will say though, that a lot of fiat casinos online have good bonuses for first-time players and made a modest sum 3 years ago when I started opening accounts on a lot of UK bookies like Betfair. The bonuses were either free wagers or enhanced odds (I always went for the latter)... when Liverpool had their good run in the Europa League I won a series of x20 bets on each win. Rolled over the free wagers and won back real money.
In the end, what serves you long term are the best odds (which is why I stick to my sportsbooks) and cashback programmes.
Although sportsbooks in the UK are an exception here, I've read in the past that people are even paying UK residents to make them accounts on those sportsbooks operating in the UK exactly because of what you said. I am not sure why it is easier to hit the bonus requirements in the UK than other countries but that's something we can say as an outlier here. Aside from what you stated, all bonuses are just something to trick people in believing they are going to get free money by betting.