Not gonna lie, why should the mc even kiss her ? "They already kissed", you mean SHE kissed him. This has not been mutual yet. He's obviously not at this stage yet, even if he has affection for her.
On one hand, i understand thay the rage stems from all the other mangas and manhwas with neutron star dense MCs.
On the other hand, y'all are completely ignoring what the mc wants. Maybe he's not ready for kisses (ESPECIALLY ON A FUCKING KISS CAM. Like holy, even married couple avoid that sometimes...). Maybe he stills wants to know her a little better, or take it a little slower. If it was the fmc doing this or saying no, nobody would be batting an eye. They sure are not at the same level of affection for each other at the very least.
Now the one thing I would agree on, is that the author should at least adress that, instead of keeping it vague. Just have the mc explain that, despite liking her, he doesn't feel ready for it to get physical yet or something.