TL,DR: this year's April's fools could be a source of confusion, or worse, in certain workplaces. Please stick to less intrusive stuff.
Very related: Why was the decision made to make the 2019 April Fool's joke immediately activate?, but this asks about the "why", and is not a feature request to simply have less of an intrusive design for next April's Fools.
Long story short, from what I gather of previous April's Fools, there was a duck, chat with an expert, overall rather discreet stuff, with the exception of Cornify (but that was 10 years ago, and SO wasn't as "big" and commonly used than today, was it?).
Emphasis on the "rather discreet", because while one can like or dislike that 90s glittery, flashy, background-repeating design, it occured to me that fortunately it's Sunday (here), and that stuff didn't pop in my office where my boss could have seen it. I knew straight away it was an April's Fools because I know SE, you know it and I know it, but my coworkers might not, and explaining will probably be more awkward than anything else.
Some workplaces are just not going to be okay with their employees browsing what looks like a 90s silly blog. Let's just not take that risk, a rubber duck is just fine...
Obviously, as said above this issue stems from the joke being enabled by default, and I do understand that making it opt-in would ruin the feel.