Chronic disease doesn't happen quickly; it takes years to develop. If you eat junk food, it won't make you diseased right away, in fact you may not feel different right away at all. All those toxic chemicals don't taste toxic. And considering the huge variety of foods you consume, it's hard to tell which ones are causing disease. And even when you do know what's unhealthy, still, because the results are delayed, it's easy to assume that each particular meal is not significant in and of itself. This is best exemplified by weight gain; even though you know what foods cause it, because it's a slow process, it's difficult to connect the dots between a particular meal and the ultimate result, or otherwise stick to a strict regimen over the long term. Healing and weight loss are the same way, they are slow processes, so once you know what diet to utilize for improvement, the improvement doesn't happen overnight, you have to stick with it for the long term to see results. So good or bad, the results come in slowly, maybe within days, weeks, months, or years.
This delayed reaction between cause and effect tempts us to be sloppy in our discipline. But the very definition of maturity and rationality is to recognize this principle of delayed reaction, and DELIBERATELY choose to use it to your advantage rather than let it be a disadvantage. Make the rational, wise, mature choice to adopt a healthy regimen permanently and consistently, and you will get the benefits in the long run.
Also, don't be swayed by narrow observation. It's easy to point to examples of people you know (or even yourself), who have lived a long life without following a healthy diet, and without suffering chronic disease, and think, does diet really matter so much after all? But what you are ignoring is that for every example of a seemingly healthy elderly person you see, there are many of their contemporaries who died all along the way, throughout all the various earlier decades, from cancer, heart disease, and other preventable diseases. The following statistics are what they are: heart disease is the leading cause of death for Americans, and cancer afflicts 40%, just to name the top killers, besides diabetes, auto-immune diseases, dementia, etc. So because such a high percentage of the population dies from preventable disease before reaching their natural lifespan, your observation of the survivors really represents a small minority, which may tempt you to assume they are not actually representative of such a small minority; you are tempted to forget or ignore the reality that the majority who already died, whom you don't think about in your analysis because they're invisible, should actually be more relevant to your analysis.