Alien life is a relatively new idea. Sure, you will find some ideas that date back a few hundred or maybe thousand years, but for the most part, it has only been a popular topic since the late 40’s or so.
I’m guessing, people have not had enough time to ask questions like this. I will need to go off topic now a little, but I am going some place with it.
I am a big fan of nonsense, comedic nonsense. I have spent a long time thinking up strange things, and have got quite good at it. When an amateur tries to create nonsensical words, they usually go for things such as “midgets, donkeys, penguins, koalas, etc”... Likewise, with aliens the first things people came up with was little people with green skin.
As you get better at coming up with nonsensical words, you start going for more mundane every day objects, and start to link them in strange ways. e.g. setting fire to children as they fold linnen. Likewise, I think it is probable that alien mythology has taken a similar path. In a better practiced attempt to come up with alien concepts, people started to take mundane things such as crops, and combining them with strange patterns and things.
As time passes, Ideas about aliens will probably start to shift towards attributing them with technology that would be a push for us to believe. e.g. being able to be in more than one place at once. Anal probes are probably a step between the technological claims and strange claims.
EDIT:
First they were green, now they are said to be grey. Grey is less different to human skin colors than green, it is an attempt to normalize in order to add credability. Limit the strangeness, but keep it a little odd.
Alien mythology is going to get much more interesting as time goes on. Recently, in Hollywood movies, they have started with monster/insect/reptile aliens, this to me would seem an attempt to move away from the humanoid form, again to add credability. Prepare for more “energy orb” claims as time goes by… people will want to move as far from humanoid shape as they can, but will eventually bring it back from non corporeal entities, to non humanoids.