It's a case of the relative amounts and proportions of harm. Gambling is something where a small but not insignificant percentage of the population have an addiction problem - they're prone to get hooked and basically lose all their money. Now, this isn't something that's absolute: someone may be susceptible to this but not actively seek it out, especially to the extent of going to a black market bookie or similar. So the fact that it's so heavily advertised is going to increase the harm to these people.
Secondly, the bookies have a strong incentive to perpetuate this harm: much like with gatcha video games, which have the exact same moral hazard (and are exactly modeled on gambling), the 'whales' who get completely addicted bring in so much more revenue than those who occasionally gamble a little for a bit of fun, that even if they're a minority of customers they can make up the majority of revenue, and so they're going to make more money if they optimize for hooking them (there are some guardrails, to be fair: in contrast to said video games, there is a system where you can ban yourself from all the major bookies if you realize you have a problem. Probably doesn't make the ads any easier to sit through, though).
I'm not in favor of banning gambling altogether: I realise it's something a lot of people can engage with healthily, even if I have no interest in it at all. But I would be in favor of reigning in the advertising of it: it's kind of obnoxious how large a percentage of it is present in sports media anyway, even without the harms done. (I feel the same about alcohol ads, even if that is something I do like, and alcohol has a much lower moral hazard from the manufacturers in terms of hooking in alcoholics, because the ratios are not so extreme)