Because it means that you have to consume more to get the same high. Which translates into spending more $$, and, more importantly, to a greater negative impact on your health.
For the longest time I was very good with one strong Weyerbacher beer per evening.
Somehow, during the winter this habit has increased to two strong beers almost every evening. I need to draw a line here and maybe get back to my original habit. I really do not want to consume this many calories and this much alcohol every evening.