well, a) apparently they used them in the past, just like aluminum bikes, and b) if they really wanted to, they could just rebrand a custom made titanium frame with their logo on it, which has literally also happened in the past. it even occasionally happens now, vis- the fake "Merckx" frames being repainted Ridleys.
like I'm sure titanium bikes are lovely. I just can't get past the whole "what the bike manufacturers DON'T want you to know" thing. Giant and Merida manufacture most of the pro bikes anyway, there's no reason someone couldn't just contract a titanium manufacturer in the same way that Colnago/Ridley/etc do so for their carbon frames.