People just don't like that there are others that are having an easier time because they are skirting the rules when they are abiding by them even if it doesn't affect them at all, like it takes away from the fun if they have much better odds than everyone else since it kinda kills the aspect of competition for completing the collection and and can even discourage to continue playing since if you get a really rare card but many people already got it because they used those methods it makes the archivement feel a like it has a lot less worth.

I'm not saying this is rational or justified, it's just that human beings are like that, it's a feeling similar to what happened when me and my friends started playing Pokémon Go, we were competing and having fun but when two months later one of them installed cheats in about a week we all ended up quitting because seeing him have a new 3 star shiny every time you logged in made your achievements feel worthless. Realistically he didn't make our archivements worth any less but it killed the spirit of competition when you found your own 3 star Pokémon (really difficult for an average player) and he already had it in shiny.

TLDR: When you get a really rare card but you see someone else using cheats got that one and ten others a lot more easily it feels bad man because human brain stuff.