I am curious why there aren’t any endoparasitic mammals, reptiles or birds
Endoparasite being a parasite that live In the host whereas ectoparasite lives On the host
I hope this clears up confusion
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Short answer size.
endo/ectoparasites need to be small to go unnoticed, and be hard to eliminate. The smallest mammals and birds are still much larger than a flea, fluke or other endoparasite. That's why larger parasites leave after feeding, if a vampire bat hung on the cow indefinitely the cow would eventually notice and do something, something as simple as rolling over could kill them quite effectively. While an animals infested with fleas can not do much to get rid of the fleas. The few large endoparasites either evolved from much smaller parasites (tapeworms) or exploit some strange insensitivity in the host(cymothoa) This means there is no path to ectoparasite which could lead to endoparasite.
Why there are endoparasites evolved by other means in vertebrates what exactly would they evolve from, there are no birds or mammals that would survive being cut off from outside oxygen or passing through a digestive system. It is like asking why no flying trees have evolved there just is no path to get there or from where they are now, or if there is a path it is so unlikely that it has not occured.