Because chcu is dialectal (from Moravia) and not standard Czech.
Ahh thank you, that’s what I know because that’s where my family’s from
I am really not sure when to use tamt... or tamhl... or the other demo forms. Do they change acoording to gender and number?
Yes, the basic forms of the demonstrative pronouns do change based on gender, number, and case. See, for example: https://en.wikipedia.org/wiki/Czech_declension#Demonstrative_pronouns. But the tam-, -hle and -to "add-ons,""when they're used, remain the same. (By which I mean to say that those bits themselves don't change, only the forms to which they're added change.)