Do you really see tele-shaped basses everywhere? LP-shaped basses? SG-shaped basses? They exist, sure--but none of those are particularly common, and for good reason. Those are all originally guitar models, and the top horns on those shapes are short (or non-existent), which can cause severe neck dive in bass guitars (due to longer scale, heavier necks). So, one of the answers to your question is that the traditional shapes are often more playable than the oddball shapes.
Also, there are a LOT of different bass models and designs represented by the brands you list. Ibanez alone has a bunch of models that look nothing like Fenders, and nothing like eachother (BTB, BTB single cut, Talman, SR, etc.). I suggest going on sweetwater or musiciansfriend or wherever and taking a peek at the huge variety of different bass shapes available at every price point. It sounds like you might be surprised!
If your question is why Fender shapes are so popular, that's pretty much common sense (hint: the same reason every consumer product in existence has a "standard" design, with other options being less popular--think vacuum cleaners, teapots, toilets, 4-door sedans, fly swatters, literally everything). People like to buy things they know are tried and true. A cynic might say that is herd mentality, but a friendlier view might be that people buy things they KNOW will work, instead of gambling their money on an unusual item that might end up having a design flaw.
