and that is....? Because their stuff "isn't good enough for trad publishing"?
These days trad vs independent aren't an if-else type question, now it's a fairly even handed option either way. Contrary to the big dream, most people getting trad contracts are not able to just kick back and watch the money flow in, instead it's a full on marketing drive that -you- have to organise and perform (while also writing more to comply with the contract), it's no free ride. For the most part, people being signed up by the Big-6 end up worse off than if they take the independent route, assuming they put in the same level of work.
Things have changed a lot in the last 2~3 years and major publishers are having to now fight harder to remain relevant among their back catalogues. There are several independent writers who have been picked up by trad-publishing, but that's only after they've succeeded as indies and are subsequently given some fairly nice contracts - unlike the ones you'll get if you start from the bottom. Conversely, there's a lot of writers who have now cancelled their publishing contracts and reacquired the rights to their work and are going independent. Trad publishers used to be the only practical way of getting your resources for making a good book (editing, proofing, artwork, marketing) but with the introduction of ebooks, print-on-demand and improved internet connectivity those old exclusivity barriers have come right down.
No matter which way you go - you have to gain your own readership. If you want that, you then take the time to do your covers well, you get your work edited, proofed, beta'd and marketed. For sure there's a lot of trashy work out there by people, but there's also a lot of professionally run independent publishing who go through all the same steps, the difference simply is that they're taking it on their own back.