The reason is because match play sucks, especially on the last day. There is nothing to watch. There is no drama. There are no runs up the leaderboard. It's just ponderous.
edit - but you are right, the knock out rounds are great. But it's backwards and that always results in a non compelling finish.
Match play itself is great. And it doesn't always result in a non-compelling finish - that Jason Day vs. Victor Dubuisson final a few years ago had one of the greatest finales of any PGA Tour event I've ever seen. The issue is that it's not made for television.
I've pondered all sorts of solutions in the past for how you could make head-to-head golf work for TV. For starters, I think the bare minimum number of groups you need on the course on the final day is four; there's too much dead time with just four players in two groups. You could get there in several ways:
1) Have more consolation matches. There are stroke play events like the Stenson-Mickelson Open Championship duel a few years ago in which you know there's a 99% chance the winner is going to be one of two players from the outset, but just having those other shots to watch and see who will finish 3rd, 4th, 5th etc. helps fill the dead air and make the broadcast palatable. So everyone in the top 8 or 12 or whomever keep playing until the end.
2) Have match play up until 8 or 12 or 16 (etc.) players qualify for a final-round stroke play shootout, with everyone starting on the same score (which is what The International, the modified stableford event at Castle Pines in Colorado, used to do back in the day) - or, after a group stage like we had this week, give anyone with a perfect 3 points a one-shot lead over everyone with 2.5 points, a two-shot lead over everyone with 2 points, and a three-shot lead over anyone sneaking through with 1.5 points.
3) Ditch the match play format in favor of medal-match play - i.e., head-to-head stroke play, like they used to use in US Open playoffs and at the old Dunhill Cup in St. Andrews. That keeps the head-to-head format intact but guarantees that every match will go at least 18 holes, and preserves the possibility of wild swings late in a match that could change the outcome. (And then the segue to a final-round stroke play shootout wouldn't seem so abrupt.)