"may be a matter of sponsorship"?
It's completely a matter of sponsorship. Those guys aren't riding the frames they would like to, they're riding the frames they are paid to ride and who's manufacturers/distributors have signed agreements to such.
Even Lance, who has a long standing relationship with Trek, is riding a frame that isn't necessarily his first choice. He used to decry compact geometry frames and during his career, more or less kept Trek producing frames that had more or less conventional cockpit dimensions. As soon as he retired, they changed their frames towards a more semi/compact design that is more market responsive. The result of his return, he's on one of those newer designs and now it's, "allright","not as bad as I used to think", etc.
Probably the only wide spread exception to using sponsors gear is in the area of all important contact points, saddle, shoes, bars and wrap.
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Birth Certificate, Passport, Marriage License Driver's License and Residency Permit all say I'm a Fred. I guess there's no denying it.