Because we want to believe that missing link is still lurking around? Because our ancestors look similar aliens cant look like that?
I honestly fail to see how shoehorning ET to support a myth is the simplest explanation.
We do have quite a record of hominid development, as the Prof pointed out. I am afraid that was the best explanation, but scrutinising the record and evidence has failed to produce results.
It's just a desperate straw grab to keep the myth alive. For 60 years people have been looking for bones or anything conclusive, and have failed, and miserably. Bigfoot is bogus.
I have actually argued that aliens are likely to be similar to us. I think it is a requirement for an Industrial Revolution. But I see nothing more than desperation to support the theory of Yeti being an Alien. The Yeti is a Blue Bear. Even Tenzing Norgay eventually overcame cultural traditions to realise the Yeti is a Blue Bear.
The Yeti has been explained, some refuse to believe the explanation. That is pretty much the short and tall of it. I do not know if an unknown hominid lives in Siberia, or in PNG, both have many claims, but the Aussie Version I know is bogus, and I see many parallels in the US version. I cannot see it being real either. I have seen a very good candidate for the Pendek, to me, the best possibility remaining is very slight, but the Ebu Gogo take that crown as "most likely undiscovered hominid to exist" IMHO.
Prof Buzzkill mentioned Zana from Siberia, that is a fascinating tale, we may never see Zana remains and therefore never test them, but we did find her sons skull, and whilst is displayed unusually robust features, it was decidedly human.