It was true initially - that third party makers brought generally cheap, low-quality lenses to the EF mount. But Sigma brought some really nice lenses to the party a little later on, with the ART series. And their 150-600 lens was really never met head-on by Canon. Nikon has/had something of that range and price, but Canon didn't counter it, basically ceding the lower-cost birding market to Sigma (and Tamron). Now, we DO have reasonably-priced super-telephoto zooms like the 200-800 (which I have sitting about 20 feet from me, having just bought one).
I think Canon actually invited Sigma and others to play with at least the APS-C lenses, which is maybe why Canon has not really brought out much in that format other than very cheap, light "kit" type lenses. Although the 18-150 is pretty good optically, for such a wide-ranging zoom, they have no f/2.8 glass and no 15 or 16-xx lenses that would mimic the full frame 24-xx "normal" lenses.