These Late English signs seem to say the tomb is... cursed? They were trying to contain something evil. All the scouts we send in fall ill and die within days.

The ancient stuff that survived to the modern day are not more durable than contemporary engineering

Basically any stone structure made for any reason will vastly outlast any steel reinforced concrete structure. Although concrete might appear superficially stone-like and unchanging it is actually porous and chemically active. Within about 100 years the steel rebar inside a concrete structure will rust, expand, and crack the concrete apart. Freeze-thaw cycles and plant activity will reduce it to rubble shortly thereafter.

Meanwhile a piece of stone block was already about a billion years old before it was cut out of the ground. A stone structure might be destroyed by earthquakes or human activity, but it does not have a built-in self destruct sequence countdown timer like SRC does.

The problem isn’t that we can’t build something that will last a millennium, it’s that we rarely, if ever, need things to last that long.

We absolutely can and sometimes we do.