People build thousands of skills in their lives—chopping vegetables, reading books, handwriting, making a budget, etc. These skills usually don’t improve linearly over a person’s life. Instead, people focus on a skill for some initial period, then it reaches some “good enough” plateau, then it mostly stays there. Some significant later event might cause the skill to suddenly start improving again, but it’s generally a punctuated equilibrium.
This happens because Naive approaches to practice rapidly plateau. Once that plateau is reached, it takes renewed effort to keep making progress: Performance plateaus often require a change in approach to surmount. At that point, the extra effort may not be worth it, or it might not be clear how to improve. Or the possibility of improvement may not be salient: Salience of improvement drives skill development.
Per Thorndike (1921, p. 178):
The main reason why we write slowly and illegibly, add slowly and with frequent errors, delay our answers to simple questions and our easy decisions between courses of action, … forget people’s names and our own engagements, lose our tempers, and the like, is not that we are doing the best that we are capable of in that particular. It is that we have too many other improvements to make, or do not know how to direct our practice, or do not really care enough about improving, or some mixture of these three conditions.
It’s rarely the case that people are anywhere near their limit, or even that marginal improvement is terribly difficult. One fun example he cites: Aschaffenburg (1896) administered daily speed tests to experienced type-setters, and without any other inducement, observed them improve their speed each day.
One big manifestation of this observation: Athletes and musicians pursue virtuosity in fundamental skills much more rigorously than knowledge workers do.
Aschaffenburg, G. (1896). Praktische Arbeit unter Alkoholwirking Work under the influence of alcohol. Psychologische Arbeit, 1, 608–626
Ericsson, K. A., Krampe, R. T., & Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363. Ericsson et al - The Role of Deliberate Practice in the Acquisition of Expert Performance
In their classic studies of Morse Code operators, Bryan and Harter (1897, 1899) identified plateaus in skill acquisition, when for long periods subjects seemed unable to attain further improvements. (p. 365)
Thorndike, E. (1921). The Psychology of Learning. Teachers College, Columbia University.