>>1245753Short version: because it doesn't need it.
Longer version:
TGVs have 2 Pantographs per power car, one for AC (French 25kV 50Hz and, on relevant sets, Germanic 16kV 16.7Hz) and one for DC (French 1.5kV and, again on relevant sets, Italian 3kV). Let's forget about running under foreign overhead for now.
Running DC (first wave of electrification, mainly in southern and eastern France), your regular TGV (Réseaux, first gen Duplexes...) can draw up to 3.7 MW worth of continuous power. 3.7MW under 1.5kV means you'll end up with an amperage just a tad under 2,500 A. That's too much for a single pantograph, so DC running is always with the DC pantograph raised on both power cars.
Under 25kV AC overhead (second wave of electrification in eastern and northern France + Brittany and high speed routes), that continuous power jumps up to 8.8MW (later TGV gens up that to 9.2MW), 8.8MW / 25kV is about 350A and you can definitely channel that through a single pantograph (and there's a high voltage line running along the roof of the set connecting both power cars).
And on top of the electrical side of physics, there's the issue of pantograph/overhead line interaction, namely, oscillation of the overhead wire due to the pantograph pushing on it. Running at high speed with both pantographs up would more or less double the stress applied to the whole wire/pantograph system and you don't really want that.
And as a side note, conventional loc&carriages sets operating under 1.5kV DC will usually run with only one pantograph up (the norm is to run with the back pantograph up on single voltage locs... or whichever is the DC pantograph on dual voltage locs) but many drivers tend to pop up the second pantograph upon leaving stations, effectively doubling the contact patch area to help with the high amperage upon take-off and usually dropping that second panto once they reach 5~10 km/h.
