There are multiple treatments for ADHD, including alpha-2 receptor agonists and norepinephrine reuptake inhibitors. Some of them show patterns of increasing efficacy out to a year (the length of the study).

The reason amphetamines are used for ADHD but not depression is that they've been studied to show that the ADHD improving effect can remain for many months, while the mood-improving effect will taper off quickly if you take them every day. Almost everyone who takes ADHD stimulant, feels a mood and motivation boost ("so happy I could cry" is the common phrase) and then is disappointed when that mood boost stops happening after a few weeks or months will learn this. Attention enhancement is less prone to tolerance, though it still accumulates tolerance too. There are some studies showing that the effects of stimulants in ADHD diminish substantially on a multi-year time frame, and it's probably not a coincidence that many people (though not all) who take stimulants discontinue after several years.