OK great, how much do you think is necessary to spend? In the 1990s the billionaire Ed Bass spent $150 million to operate Biosphere 2 with an expected mission time of two years. Even after spending $150 million in 1993 dollars, the experiment shut down after only 6 months. [0]
So let's say we spend a little more money ($150 million would be approx $335 million today, so maybe $0.5 - 1 billion). That's a lot of money but still a LOT less than establishing a moon base. And by doing it on Earth we can solve for more closed-loop system problems here without also coping at the exact same time with an environment actively trying to kill the participants and a multi-month lead time for any resupplies/"oops"es.
Then we can take those lessons learned and apply them to the much harsher environment of the moon. In other words, it's a lot cheaper and faster to learn some lessons here on Earth first before having to tackle all that plus things like keeping yourself alive.
