We have a certain amount of "willpower" that can overcome motivational barriers. This willpower is limited, though. It's very helpful to have clear guidance on where to allocate this very limited resources, especially in situations where people are otherwise struggling.
All of which is to say -- these exhortations can play a useful role. :)
It's also worth noting that willpower in general is constantly being whittled down by how stressful and, for lack of better term, fucking annoying modern life is. I'm reminded of a quote from my favorite article from Ed Zitron:
> In plain terms, everybody is being fucked with constantly in tiny little ways by most apps and services, and I believe that billions of people being fucked with at once in all of these ways has profound psychological and social consequences that we’re not meaningfully discussing.
And I think one of those psycho-social consequences we're not discussing is everyone is just... fucking annoyed now, constantly, about shit that it doesn't feel right to really complain about. Like, you plus or minus live on your phone, and I'm very much including myself in that statement. Every time you get logged out of an app you use every day to, for example, board your morning train, or park your car, or have to reset a password to pay your power bill, just like, all of that? Every time your day is interrupted with stupid bullshit from Modern Life takes a tiny bit of that energy, and I dunno about everyone reading this, I have a quite well managed and streamlined life, and I still have just... dozens of these. Every single day. I can't fathom being one of the folks who ISN'T as well versed in tech as I am, existing for them must be utter HELL.
And that's the essentials, that's not even going into how most tech products now are constantly begging for your attention, for your engagement, trying to pluck the strings of your psyche into making you angry, or horny, or whatever. Engage with platforms, buy these products, watch 9 TV shows so you're not out of the loop, you've been added to an SMS spam group, and everyone is replying to it saying they're not interested, on and on and on.
Sorry this turned into more of a rant than I really envisioned but yeah. I can easily comprehend a day where I try and go to my gym, and the fucking app doesn't work right and I can't get in, and I just quit because I've already solved 20 fucking captchas today and I simply lack the energy to do another, to help train some goddamned AI, for a company I don't know, you know?
Yup, which is why diets fail. People who are fat generally do not have enough available willpower to lose weight.
AIUI, the whole "certain amount of willpower" was a misunderstanding / exaggeration / bullshit claim that's not really held to be true anymore.