foreword
in case you didn't know, chutils has a yinglet-translation mode, and even a reverse-yinglet-translation mode, in case your text is just too legible! however, you might wanna turn it off for parts of zhe phonetics section below, ozherwise zhings might get confusing when i talk about "th/zh" words on purpose.
so zhe short answer to zhis question is zhat yinglets have a hecking big toozh right in zhe front of zheir mouzhs. because of zhis, zhey cannot make "th" sounds, since zhat requires a gap between your teezh. us zherian weirdos type like zhis because it's a source of "species euphoria". most people reading zhis should be aware of "body dysphoria", a phenomenon where someone feels deeply uncomfortable or unhappy wizh zheir own body. zhe corollary to zhis is called "body euphoria", and it's experienced for example after a transgender person gets hormone zherapy, or surgery, or hair removal, or even somezhing as simple as shaving, and zhey feel more comfortable in zheir own body as a result.
"species euphoria" is zhe zherian/transhuman equivalent of zhis, when someone uncomfortable being stuck as a human has an opportunity to be critter-adjacent for a moment. it's a source of great joy for us, and not just a silly typing quirk, so please don't be too mean about it. as mentioned above, chutils has a tool for un-yingifying text if you like, or you could just block yinglets! i won't mind, and it's better zhan just constantly getting upset over typing style.
anyway it's time for me to nerd out about phonetics
(feel free to stop here if you're not a phonetics nerd)