How come decades in the 1900s look fairly well differentiated but from like 2004 on feels like a giant run on?

I understand not every decade was a clean break, but each decade has fairly distinct defining properties. It feels like most of the 21st century has been a single run on with smaller changes. I know sometimes the definitions don’t come into focus until later, but I’ve been around since the 80s and I can distinctly remember the changes between the 80s, 90s and 00s as they were happening.