I don't think teams tank as much they are claimed to do so, probably because it is not as beneficial of a strategy as one might think.
Let me first define what I mean by "tanking." Tanking in this context means deliberately losing on purpose to get the best possible draft pick. I do NOT consider tanking to be the same thing as a rebuild, where you do not expect to get good enough value for the salary, and so you trade away assets or simply let them walk in order to bring in younger and cheaper talent to develop. In doing so, you accept poorer performance along the way until you can develop that talent. When I ask about tanking I'm asking if teams not only do that, but also lose on purpose to improve their draft position.
First, research has shown that "top draft picks are significantly overvalued in a manner that is inconsistent with rational expectations and efficient markets, and consistent with psychological research." The basic reason is that there is a salary cap. No one can outspend any one else, and so you have to extract as much value for a fixed amount of money as possible. Top draft picks are better players than later picks, but they also command much higher salaries that tend to overcompensate them relative to others. The best value performance combo actually peaks in the LATE first round, and all picks after the first overall are on average better value until the early third round.
https://d3i71xaburhd42.cloudfront.net/61bbf4dd4aeb2e915f631832dc890f92a9a0c12c/58-FigureIV-1.png
source: 10.1287/mnsc.1120.1657
This has been dubbed "the loser's curse" because bad teams tend to pick good players but pay them too much, they cannot pay sufficient talent elsewhere on the field, and the team continues to perform poorly. And when you remove QBs from the calculation, this phenomenon becomes even more pronounced, because QBs are such high value and tend to be more reliable performers.
https://opensourcefootball.com/posts/2023-02-23-nfl-draft-value-chart/moo.png
source: https://opensourcefootball.com/posts/2023-02-23-nfl-draft-value-chart/
So it's better to just be mediocre or even good but not great. With the exception of an elite QB propsect, losing on purpose to go as high as possible in the draft is clearly a bad strategy because you are probably going to overpay. Even if there is an elite QB, the hype around that prospect could improve that player's negotiating position for him to demand more salary from you, increasing the risk of overpay.
Second, there is a cost to tanking. We as fans are only looking at the team as a whole. It may help the team to get a (nominally) better draft position by losing. But a team is composed of individual professional players and coaches, all of whom hope to continued to be employed as such. Losing winnable games certainly does not aid them in this pursuit. Even if losing on purpose was a good strategy, is it actually possible to get players to follow it, since it requires them to play worse deliberately, or frustrate the efforts of those who still play well? And how do you develop and maintain a winning culture and attitude while losing on purpose? Tanking is likely to cause you to alienate and waste the development of current talent, and fail to attract better talent from elsewhere in the league - all in favor of gaining an uncertain and likely to be overpaid draft pick.
Finally, I'm aware of only one situation in which someone inside an organization explicitly claimed that there was an aim to lose on purpose - Brian Flores's lawsuit, in which he claimed the Dolphins owner offered to pay him to lose. I don't know if these allegations were ever proven, but if true they are still illustrative - Brian Flores was so insulted by this offer he started a lawsuit (among other reasons). Deliberately losing is anathema to every single person who has poured a lifetime of blood, sweat, and tears into being a professional coach or player. I don't think enough players or coaches would accept it to begin with, simply because they are too disgusted by the idea.
In conclusion, I think the popular beliefs that this or that team are tanking derives from the disbelief that a team can really be as bad as it is. The truth is it's very difficult to win professional football games, so hard that some pros look like they are doing it on purpose. Especially because we love to be armchair QBs, HCs, and GMs, who think we know what a team should do, when in fact the vast majority of fans simply have no idea what they are talking about. And for fans of a specific bad team, it is copium. It makes you feel better to think your team has a strategy, when in fact they just suck this year, and may suck for the foreseeable future.
TL;DR - Although teams of course do rebuild, I don't think teams really tank, with perhaps rare exception of which I am not aware. It's questionable whether tanking actually works, and I don't think it's likely you'll get the players and coaches to actually sign on for a tanking campaign.