Nominal value is not real value
A bonus is not money until it can be withdrawn. Between the two sits the wagering requirement, and that is where most of the headline number goes.
The arithmetic, written out
Take a deposit of 100 with a 100% bonus and 40x wagering on the bonus alone. You must stake 4,000 before the balance is yours. At 96% RTP the expected loss on that turnover is roughly 160 - more than the bonus was worth.
What to compare instead
The multiplier, what it applies to, how games are weighted and the maximum cash-out. A smaller bonus at 20x beats a larger one at 50x almost every time.