The AR is designed and built for modern manufacturing, where we had NC controlled machine tools (the prior generation to CNC automation). Bolt and lever action rifles were designed mostly in the pre-NC/CNC era, and they require more machining setups.

The bolt action rifle that was designed to be made for cheap(er) manufacturing is the Savage 110 and similar rifles which use a barrel nut to control the headspacing of the barrel against the bolt. The second bolt action rifle that was designed for manufacturing ease and lower costs was the Remington 700, which has a tube of 4140 steel as the receiver. The headspace can be adjusted by surface grinding the recoil lug (which is also a spacing washer under the barrel) to bring the barrel into headspace requirements.

The original bolt action rifle that really made a huge impact in rifle design, the Mauser 98, is still available from custom rifle action builders... for a few thousand dollars for the action, bolt, and bottom metal. That's just the action, not the barrel or stock. That's how much machining is involved in a Mauser action.

In AR's, there are only two critical machining operation in the entire rifle: The first is the length of the barrel tenon, onto which the barrel extension screws and is pinned. The second is the depth of the chamber pocket on the face of the bolt.  These are what controls your headspace on the AR design rifles. Typically, since you don't machine your own bolt head on an AR, once you have the barrel tenon machined to the correct length, you screw on a barrel extension, torque it to the correct spec, use a bolt to check the headspace for operation with a "go" gage, and you're done. Everything else on the AR is very simple assembly.