Mountain bikes have to be lightweight and strong, and production volume is low. Suspension design takes R&D, and adds moving parts. Start pricing components and you hit $5000 easy for a full-suspension bike. For hardtails, you are making a lot of compromises at $1500, but $2500 gets you a nice bike.

For road/gravel bikes, once you get over $2000, you are paying a lot of money for tiny weight savings, negligible aerodynamic improvements, and electronic gizmos.

For either mountain or road, if you want a custom/hand-made frame and parts made in the developed world paying living wages, you are going to spend a lot more. Taiwan makes a lot of great frames, but if you want a frame made buy a dude in Denver who names all his bikes after craft beers, add several grand.

For city/commuter bikes, you can get something perfectly good for under $1000, but if you can swing $2000, get a Brompton.