Very few people tune in on Monday during a US Open playoff. Volunteers who've devoted 4 days of work can't be expected to miss work that following Monday to support the tournament. Viewers who have tuned in for the weekend, most of them, don't have the luxury of staying home Monday and watching the telecast. The people in attendance on Sunday, most of them, don't have the luxury of staying an extra day, postponing flights and extending hotel stays, to watch an 18-hole playoff on Monday.
I think the British Open is a great example of why a 4-hole playoff based on aggregate scoring works. People continue to watch, it doesn't take forever and a day to complete, and people stay interested.
And it's most practical.