Nitrates aren't the only reason regular water changes are recommended...they're just one of the easiest and cheapest to test for. Without water changes, your fish tank is a completely closed system...nothing that enters it leaves it. This means, without water changes, the concentration of minerals, bi-products from natural reactions (like the nitrogen cycle), and such will continue to grow if you don't take them out. That also means the trace substances in your water that ARE being used get used up and even with evaporation top-offs, you don't always replace them as fast as they're being used.
There's several things that are recommended to reduce the needed frequency of water changes...from picking a manageable stocking level, water volume (using sumps and external filters to increase it, for example), adding plants and such to increase the depth of the biological processes in the closed system and to attempt to fill in missing parts of the "circle of life" found in larger systems found in nature, etc.
In the end, it's all a stop-gap to reduce the need for opening the closed system with water changes, not eliminate the need...and for years (probably close to a decade or more now that I think of it) the "best practices" have suggested weekly to bi-weekly water change schedules (many choosing to do even more regardless) for optimal healthiness, and when many of us read that it isn't happening, we wonder what's going on in the tank that could impact it negatively, both short and long term.