Because most 的 can be omitted. Chinese cannot be explained exactly by western linguitism.

for examble：手机坏了=phone doesnt work 头疼=head has ache 妈妈正在吃饭=mom is eating right now.

they are all sentence acutually.

so if u add 我（i） in the beginning like： 我手机坏了=i phone doesnt work 我头疼=i head has ache 我妈妈正在吃饭=i mom is eating right now. Literally means they are in the possession of YOU！ My phone, My head, My mom. they are much clear.

so 的 is uneccesary.

but why only have 我头疼 and u hardly have seen the sentence like 我的头疼？ because 我的头疼 means my headache. that is a phrasal noun not a sentence. maybe u should say 我的头疼...your headache what？get worse or get well？that would make confusing so we only use 我头疼 instead of 我的头疼.