The difference is in the details, that with other paid DLC, you actually get the thing you paid for, guaranteed. With a gacha, if they’re promoting some super-strong character, weapon, etc. that you want and you buy currency to spend in the gacha, you are not guaranteed to get that item or anything of the same quality/rarity in any of those pulls you make. It’s all random chance, gambling at its core. Exceptionally good or bad luck can start playing psychological tricks on you, such as FOMO (there will always be something stronger coming soon), sunk cost fallacy (you’ve already dumped this much into it and got nothing, what’s the difference with this much more?), and before you know it, if you’re not watching carefully, you’ve spent far more in in-game and/or real money than you realized. That’s far different than a one-time purchase straight-up for a cosmetic or weapon to use with no further need to spend any more, and that’s what gets people hooked like gambling. You may not have experienced this much because gachas tend to be very generous to new players in order to get them started out quickly as whales fodder and get them hooked on the adrenaline rush of “winning” in the gacha system before the gacha currency starts to dry up on them.
Most “pity” systems require hundreds of pulls beforehand, which unless someone saves months worth of free currency for those pulls, can be very expensive in real world money to get the currency to afford. In a way, pity systems are just designed to increase the amount of money players spend.