A good amount of modern building code is related to energy efficiency. These standards applying to new construction make sense: it's only a marginal increase in cost to add insulation, use upgraded products, or ensure air-tightness during construction. However, for existing homes, this would be extremely costly. The house would, in many cases, need to be ripped down to the studs on exterior walls and completely reinsulated/sealed, older windows would need to be replaced, HVAC ducts exposed and sealed/insulated, etc.

For an existing structure, the result of all these improvements, even considering rising energy costs, would never pay back over the expected life of the house. Code tries to strike a balance here by enforcing that when it's reasonable to do so (e.g. you tore down to the studs for a remodel anyway), you need to bring the areas modified up to modern-ish standards.

That's fine where the benefits of code compliance outweigh the costs of compliance.

But old homes are expensive to rewire. And often, the safety benefits are very small. Current code requires a lot more outlets than older code, in part because many people overuse extension cords and tiny/cheap power taps when there aren't sufficient outlets, which is a safety issue, but is it worth spending thousands of dollars to pull new wire through existing walls, and then repairing the walls afterwards?

Some issues, are worth retrofitting for, and hopefully a pre-sales inspection by the buyer or insurance inspection will catch those. It would be reasonable to have a transfer inspection for those too.

To bring something up to code, it takes weeks if not months. Getting permit, construction and inspection. You can't tie up a sale until that's done. Where do you get the money to pay for that? Buyer isn't gonna wait for X months, especially if buyers want to do their own renovations. You know what people will do? Pass the buck to the buyer (Sold As Is). The cycle repeats again and again.

My house is 40 years old. To "bring it up to code" would have one simple step: demolish the whole house. There is no way you will ever economically bring a structure this old up to code.