Mine only goes half to 2/3s down the glans when erect, and there is enough slack to where if I hold the foreskin lower down it can slide up to mostly cover it. Sometimes a lil bit of spit or some such is still needed to lubricate the glans though.
Broadly it's probably just difference in genetics of size/slackness of foreskin, idk
Probably some combination of not necessarily producing enough (not all people make the same amount) and lack of foreplay/edging
personal anatomical anecdote
For example I really only have precum after a decent amount of foreplay/edging, and not all that much. And a lil lubrication is sometimes useful for getting to that point.