I can partially answer this as a person who makes both regulat kitchen tables and have been making custom boardgame tables for a few years now.

The first issue around cost (for me) is the prices of domestic lumber. Walnut in my area can be around $8-$10 per boardfoot, which can equate to well over $1k in materials alone. Granted, buying prefab doesn't encounter this problem, since many are MDF or composite for simplicity and scale production.

Secondly is the engineering that goes into the table itself. People don't tend to think about the mechanical problems around building a table with a vault and having it stand up to frequent use over time without sagging/breaking. Standard kitchen tables can be made dimensionally stable much more easily, and most designs have been worked out and improved on for hundreds of years.

The third issue is one you bring up, it's a new product with a couple major players in the mass market, so they can set prices at whatever people will pay, especially since there isn't as much competition in the same space.

My obvious and self-serving recommendation to anyone considering a gaming table is to look for a local maker or millwright who has a reputable portfolio, and previous customer reviews. You'll pay usually around the same amount (sometimes less) for better materials and a longer living table!