The Introduction of magic into a D&D Economy

[Jack_Simth]

Two reasons. One, population doesn't expand really in post industrial nations (the US is ahead of the curve and is just barely at replacement level, most of the EU is shrinking). And two, security. The reason the people are in the cities is because the land outside of the cities can't be defended and isn't safe (and can't be made safe).

Nothing can be perfectly safe. For every defensive measure, there is a countermeasure. The most you can do is make it expensive to attack you... but that is, in and of itself, very expensive. Ultimately, all of society relies on trust to one degree or another.

Forbiddance can not protect a nation. You might be able to cover a city but the price tag runs 252,500 GP to cover a mile of land. And that only provides cover up to 60 feet up. Getting into an area protected by Forbiddance is as simple has having a guy go and rent a room before digging down 10 feet (they are now below the spells level) or just teleporting in above it.

And you can't defend a shoreline, either. This is a classic beachhead scenario, really. As for your cost references....

Just for reference it would cost 2,525,000,000 GP (that's 2.525 BILLION) to cover 100 square miles of land to a height of 60 feet.

A Tippyverse mostly relies on the ability to make spell effects at-will anyway. While yes, it would cost about that to do repeated castings, a Command-word Widget of Forbiddance, at the minimum caster level (11), would run (by DMG estimates) Spell level * caster level * 1800 + 100 * material component cost. If we set it for a single 60-foot cube per invocation, with the password option, that's 500,000 for the material components. Grand total for the widget: 618,800 gp (market).

Compare to the True Resurrection trap you postulated for dealing with the XP problem, which runs 2,500,000 for the material components, 76,500 for the magical supplies, and 6,120 xp to craft.