I don't think we decide not to get better in any conscious sense.
Certainly is a red flag word though, isn't it?
Instead, I think we carefully avoid deciding at all, and that's half of why we aren't better. Those of us who do get better, except for the occasional Mozart, are those who did consciously decide.
I believe Chang, Bernhard, and others are correct in that our natural intuitive methods for learning are inefficient, counterproductive, and wrong. Therefore if we don't "decide" to be good but just muddle through, it is unlikely to happen. Even if we do decide to put in the effort, it has to be intelligent effort, and for that you need good external information.
Time is a factor, certainly. Most of us could organize our day rigorously and have plenty of time to practice, work out, learn Sanskrit, date supermodels, and raise a family in a spotless house. However, the degree of organization that would be required is in itself aversive. Hee, hee.
Even with practice, the effective methods are not as much fun as stumbling through. I see that when I sit down to practice. Last night I was really tired after work, and have some stuff for church that needs to be ready. The best use of my time would have been to master four measures, or maybe two or one and go to bed. But i didn't feel like doing that, didn't want to force myself to concentrate that hard.