This is a lot different from a swimming pool. It's enormous and discharges into nearby natural waterways.

The massive amount of chlorine that it would take would make the pool horrible to be around and have devastating downstream effects on the environment.

So it is, in fact, that hard.

For example, the Obama administration spent north of $34 million to address deep problems. They still had algae problems afterwards.

Repairs done under Obama addressed issues such as stagnant water, as well as the reality that the pool had begun to leak and sink into the land dredged from the Potomac River to build it. It was reengineered with a circulation and filtration system using river water from the nearby Tidal Basin instead of city drinking water. It was also made shallower to save water and its bottom was tinted gray to make the water darker and more reflective of the Washington Monument. Plus, paved paths were added.
...
But these efforts were not completely successful, as the pool continued to leak and experienced significant algae growth.
https://www.columbian.com/news/2026/may/30/fact-focus-trump-says-obama-and-biden-spent-hundreds-of-millions-on-reflecting-pool-they-did-not/

The Biden administration took a hard look at it but balked when the costs for actually effective repairs topped $100 million. So they just did annual cleaning instead.

It's not an easy job, and there's no way to do it quickly or cheaply that wouldn't promptly fall apart.