Think about the amount of time, energy, and effort someone has put into his or her unhealthy behaviors and lifestyle. It's generally a lifetime, hundreds of thousands or even millions or tens of millions of dollars, and the entirety of his or her being. Objectively and relatively, that is A LOT of investing. Thus, people proffer nonsense equivocations about changing their lives: "I can't afford the time or the money for this gym membership, healthy food, workout, coaching, etc." That isn't really what's going on, because it is a pittance in comparison to the years or decades spent on becoming and persisting in unhealthiness. The fractional costs involved in becoming more fit or healthy are non-comparable to the immense expenses involved in getting and staying unhealthy. We all know it. What's really going on is that people have to substantiate their terrible lifestyle decisions (even while they knowingly are in the wrong) to feebly keep the ego somewhat in tact (so as to not have a complete nervous breakdown) by persisting in their bad choices and prior familiar unhealthy investments.
Sunk Cost Fallacy plays a strong role when people resist beneficial lifestyle change, new scientific discoveries, and just general life management. Apparently, it's built way down into the mammal brain. It's being studied all the time: