Fairings aren't as useful as you might think in most circumstances. On an upright bike, they need to be enormous to offer an appreciable benefit, which makes them heavy and difficult to control in crosswinds.
The optimum solution is a recumbent bike - by going feet-first, you can reduce your frontal area by more than 50%. With a much smaller frontal area and a much lower center of gravity, a tail fairing becomes a practical proposition.
Fully enclosed recumbents can achieve phenomenal results in the right conditions (Sam Whittingham's 91km hour record, Andy Wilkinson's 41 hour LEJoG), but they immediately become a handicap with any sort of gradient because of the ~20kg weight penalty and they're unbearably hot.
Recumbents have been banned by the UCI since 1934, so that ship has sailed. If we had seen recumbents on the Tour de France for the last eighty years, they'd probably look cool and upright bicycles would look weirdly old-fashioned.
Ok flat ground or a descent a recumbent may be faster (my experience is that most recumbents can’t keep 22-25mph on flat ground) but it won’t be faster climbing hills and they have a longer wheelbase and are quite a bit less maneuverable. On hill climbs the weight is the most obvious factor but I’d be surprised if you can generate the same amount of power in the supine position as you could in the standing position.
Since most bicycle racing is dominated by climbing hills, I don’t think anyone would ride a recumbent even if it was legal.
A modern short wheel base recumbent has a similar wheelbase to a conventional 700c road bike. The hill climbing issue is still the subject of some debate; your peak torque is undoubtedly lower (because you can't put your full weight on the pedals) but there's no obvious biomechanical reason for lower power.
Any advantage an upright bike might have in the mountains is completely neutralised by the tremendous aerodynamic advantage of a recumbent. A good lowracer recumbent with a tailbox has about half the drag of a normal racing bike. Worse still, an upright rider gets almost no benefit from drafting a recumbent rider.
If the UCI legalised recumbents, a team using upright bikes wouldn't even finish the first stage of a grand tour within the time cut-off. Any team using recumbents could disappear off the front of the peloton and would be impossible to catch. In a time trial, there's simply no contest - the recumbent riders could demolish the upright riders without breaking a sweat. To neutralise that advantage, races would need to be organised exclusively in the high mountains with no appreciable amount of flat roads.
Why? Is there some physiological factor? Blood flowing too much to the head? Allegedly the hydrostatic pressure is lower, breathing is easier due to lack of bending on a recumbent bike, so it should help endurance, as long as the bikes are the same weight.
The optimum solution is a recumbent bike - by going feet-first, you can reduce your frontal area by more than 50%. With a much smaller frontal area and a much lower center of gravity, a tail fairing becomes a practical proposition.
Fully enclosed recumbents can achieve phenomenal results in the right conditions (Sam Whittingham's 91km hour record, Andy Wilkinson's 41 hour LEJoG), but they immediately become a handicap with any sort of gradient because of the ~20kg weight penalty and they're unbearably hot.