Because the transporter is a nightmare fuel generator even when they are not using it to split two living beings that are literally bound by flesh, and even biologically synchronized to a high degree?

Because you should at least get a chance to be born before being replaced by a procession of short-lived transporter clones.

I'm just surprised the crew doesn't beam the shit out of their colons on the regular.

Didn't they do a teleporter birth on Voyager and it caused the kid's cell membranes to dissolve?

I like to think it's because healthy natural child birth is optimal for both the babys and the mothers wellbeing.