The sump may have become unnecessary by; changing drainage, e.g., French drains, gutters/downspouts, flow from neighboring properties; environmental changes, e.g., rain, springs/water tables.

I don't know where you live so it may or may not apply. Several years ago,20 at least, it became illegal to drain water into sewer system so at least that may be why it's capped. We had to get sump pump pit plumbed to empty outside. On the very, very rare times our goes on water is carried away to the far backyard by a garden type hose on the lawn.

Maybe the codes for how the sump pit was built is no longer up to code and cheaper to fill it in rather than repair.

What Izzy said, most codes don't allow pumps drain into the sewer it must discharge to the daylight. Also, most places require Radon Sump pump pits to be installed with Radon Cover to prevent the Radon gas from entering the house.

That being said, they probably never had water issue so they filled the hole with dirt and put some cement over it. Not a big deal to get it back to work condition.

Good luck