Standard seconds are defined based on measurable properties of a cesium atom. The historical definition of 1/86400th of a day doesn’t work for science if the duration is inconsistent.

For example the statement:

Earth’s Days Are Getting 2 seconds Longer Every 100,000 Years

becomes self-referencing and loses all meaning without some other reference point.

This is time relative to earth, and the actual passage of time in the universe that we aim to measure doesn’t care about the Earth’s rotation.