Because of the difference on bankroll. Rich people have a huge bankroll that gives them freedom to increase bet or play more until they win huge amount while poor has only limited bankroll which they will stop immediately once they lose enough.
Bankroll is part of the reason but also the media makes a big deal when a celebrity influencer wins a lot of money. We don't hear much about a whale going bust and losing millions in one night. This causes us to perceive that rich people are just winning all the time when in reality they are also the biggest losers. Having greater wealth can have certain benefits when gambling but you can also incur higher risk so it ends up balancing out where statistically the average rich person will have a similar losing percentage as someone who is poor.