Why is because most of the people who really disliked season 1 and gave it very low ratings probably didn’t bother with any further seasons. So a sequel would have to really drop the ball pretty hard for anyone who overall liked season 1 to give it a very low rating.
We see this with In Another World With My Smartphone, where even its season 2 has a higher rating than season 1. I have interacted with other Smartphone fans on Reddit and the disappointment with season 2 isn’t just me, most everyone agrees it rushed through too much too quickly and thus is a downgrade as an adaptation compared to season 1. So season 1 having a lower rating is all because of the haters of “generic’ Isekai who watched it just to clown on it and then forgot about it.
However, Smartphone season 2 only has slightly higher ratings. For sequels who have more substantial leads over earlier installments you really shouldn't dismiss that as just this phenomenon at play.
The beauty of Anime lies in part in how many saga produce their best content 3 or 4 or even 5 seasons in, or 100s of episodes in if it’s all still one MAL entry like Detective Conan. This is not an artform where the popular pectin that everything is only good early on then it runs out of dies fast applies. The shows that I do feel have a central gimmick that got stale fast couldn't keep me for even a whole season.
Rent-a-Girlfriend is a show I enjoyed even in season 1, but season 3 was a massive elevation, and a lot of the most hated qualities of the series are muted drastically during that season.
And even with Smartphone it’s an adaptation issue, I'm confident that if the content of season 2 had been fleshed out to fill 2 or 3 seasons it could have been equaled the quality of season 1 or maybe even surpassed it.