There are so few games in the NFL. The difference between a top-5 or top-10 pick and a playoff team is often 4 games, say 5-11 versus 9-7.
And, as mentioned above, the first pick in the draft is rarely a franchise changer. Since 1980, the list of franchise changing 1st overall picks is pretty low (player has to have changed their original franchise to count, so Elway doesn't make the list):
Bruce Smith
Troy Aikman
Drew Bledsoe
Orlando Pace
Peyton Manning
Michael Vick
Eli Manning
Cam Newton
Andrew Luck
And even some of those are very arguable in that they probably required other pieces (Aikman, Pace, Smith, Manning) or merely changed a franchise from a basement-dweller to an up-and-down playoff team (Bledsoe, Vick, Newton, Luck). Of them, only Peyton Manning really had a truly transformative impact, and that's not surprising because he's one of the greatest QBs of all-time. But even that required Marvin Harrison, Reggie Wayne, Edgerrin James, Dallas Clark, Dwight Freeney, Bob Sanders, and so on.
Even if we expand it to top 5 overall picks in that time period, the number of franchise changers (not good players, so there are Hall of Famers who are not here, but players who appear to have legitimately changed their franchise's fortunes in the years after they were drafted) is relatively small:
Anthony Munoz
Lawrence Taylor
Eric ****erson
Derrick Thomas
Steve McNair
Jon Ogden
Donovan McNabb
Ladainian Tomlinson
Julius Peppers
Larry Fitzgerald
Philip Rivers
Von Miller
Khalil Mack
Zeke Elliot
That's maybe 23 players total out of 185 top-5 selections since 1980, and many of these are very borderline (a stricter definition would probably remove many of the non-QBs and RBs; for instance, Rivers and Tomlinson probably shouldn't both be on here, I'd choose the latter above the former in terms of impact). The list does slightly favor number 1 selections, which are a fifth of the total and nearly a third of the "franchise changers," but not remarkably so and only because quarterbacks tend to be drafted at #1.
In any case, choosing to tank requires it to be a season-long strategy rather than something you can implement halfway through since there are so few games in the NFL. Nor does it seem that there's much to be gained by it.