there is no option to pay with paypal, why?

That is game specific. Ask on that game's comment section not in general community.

Assuming you talk about a paid game and not a free game.

People who sell games on Itch can choose to do so through Itch or sell directly.

If they sell directly, then they have control over the payment method and can choose not to accept payments through PayPal for example.

As @Redonihunter says, in those cases, you should consult directly with the seller of that game.