At the time the Founder of aikido learnt daito ryu aikijujutsu (the old art he based aikido on), almost everyone knew sumo. This has been used as a possible explanation of why there are no hip throws in aikido (well there's one but it doesn't come from daito ryu and has been developed by later generations of aikidoka) and I think that it can, at least in part, explain the lack of takedowns and takedown defences in the art.
Many students of the Founder were also good judoka thus he might not have deemed necessary to teach them such techniques, if he had ever been interested in doing so.
We have accounts that Sokaku Takeda, Morihei Ueshiba or students like Koichi Tohei were "unthrowable" by advanced judoka. The real question here is "how come?" What did those guys have that is or isn't taught in modern aikido?