That's stupid.
Why didn't they do like some of the Black and White people did and crossbreed?
Uchiha and Senju Clan members' children would be very powerful and maybe unlocking Rinnegan later in life and being genetically skilled in... Everything. It'll be like all their children will be young Nagatos. Large chakra pools, powerful bodies, and superior ocular powers. Yeah, Nagato is basically like a Uchiha/Senju cross breed.
If they raised someone like that, then the top 10 list wouldn't have anyone else in it.
the problem is that even in the uchiha and senju the only things they knew about were izanagi and izanami being benefits from that. and as those sacrafice the eyes it would be far more tempting to kill uchihas and senjus to get the requirements then to go though all the work of not only raising a child, but raising a ninja for just two eyes that your going to sacrafice. hashiramas dna was the exception, he was the one senju that wouldve been tempting to kidnap and raise an army from, but nobody knew about his life giving chackra or wood release until he was too old and too strong to do this to. the better question would be:
why didnt they kidnap just uchihas since they they all couldve had sharingan and you couldve built an army just as strong as the actaul uchiha clan.
the bottom line is noone knew about mangekyou sharingan until madara and his brother got it, and that was only 80 years before the founding of konoha, which is when ninjas suddenly develope codes of ethics and wouldnt do things like this other than kumo. and anything before madara and hashirama wouldve been much less tempting when you consider that you not only have to kidnap two memebers of the two strongest clans, but you need a boy and a girl, so you cant just kidnap the first kid you see at least not after you get the first one. and at this time the only benefit from having one senju and one uchiha would require that you sacrifice the both the eyes and the kids that you put so much effort into raising and on top of that lost possibly most of your own clan to obtain as well. it wouldve been more beneficial to make a second uchiha clan than a clan of rikudous based on the info available at that time. and then the clan in question would have to distinguish between the uchiha children to make sure one is a boy and one is a girl, which would take more time thus losing more lives. and then again you would still have to raise not just one, but an army of not only children but ninjas.