Because we all want to waste like 8-16 hours just to finally have sex with all of them. If you were to actually play the game, each character's story generally takes an hour or two to beat given that you know what you're doing and not just fucking around.
There are 8 characters in total that you can choose to pursue. 4 of them are human and 2 of them are demon goddesses that are obviously your age and attend your school and want to absorb your dick. The other 2 would be the loli demon and your teacher at school.
In the link provided shows only 5 of the character's CGs (2 humans, both demon goddesses and the loli demon). These 5 characters are primarily the main characters in both the game and anime. The other 3 (remaining 2 humans and your teacher) are side quests in the game and secondary characters in the anime.