Dopamine Level and Cognitive Ability

The article (https://neurosciencenews.com/dopamine-cognitive-ability-10788/) mentions that the right amount of dopamine in the brain will improve cognitive function. Tyrosine is a precursor for dopamine. Tyrosine is a polar amino acid.  An experiment was conducted where participants were given tyrosine juice or a placebo. Then the participants were given a memory task where they viewed a bunch of slides and took note of if there were any replicates. The participants who had an increase in dopamine levels with the tyrosine juice performed at a "faster rate with fewer mistakes". In class I learned that dopamine is responsible for attention and motivation. Participants who had the extra dopamine would have a higher level of attention and would pick up on more details. Also their motivation to complete the task would be higher too. Therefore, the level of dopamine in our system to function at a higher level is not always met. My question is if there is an optimal level of dopamine for the brain to function at that level, why is the brain not constantly producing dopamine to reach that level?