Why do they call a billboard a billboard?

What has Bill got to de with it?

Because it is actually : Bill Board

People would receive various notifications that they would refer to as 'bills'.

Today we have separated that term into multiple meanings. Such as a 'flyer'.

The board part is self explanatory I guess.

Old: I got a bill in the mail today about a sale
New: I got a flyer in the mail today about a sale

Electric Bill, Phone Bill, etc. All Bills is drawing attention to something.

Aaaahhhhh, now I get it

Thank you for this explanation

They were invented by Sir William Plank, but he didn't want anyone to think he was self obsessed, so he obfuscated it a bit.