I may be using the wrong term, but I understand the infinitive form of a verb in Japanese to be the form we add ～ます to. In the case of ichidan verbs, we take away the ～る and add ～ます、and for godan verbs we change the ending う-line to an い-line and add ～ます. Why then, is くださる not conjugated as くださります in infinitve form, and instead conjugated as くださいます? Additionally, why do we only use ください most of the time?
EDIT: To clarify, I'm interested both in the functional purpose of this, and any historical or cultural purpose of this if they exist. Thank you!