Let's say you play poker with some friends for money. In the FIRST or second game you made some good decisions but you also had extraordinary luck, and you just won 100$ from the other poker players. Immediately after the game you say "Alright guys, I think I'm going to call it a night, see ya later!
That's why pro poker tourneys play a set amount of hands.
To be clear, most professional poker is played in cash games, where there is most certainly not a set amount of hands. Tournament play is what's televised because it's more dramatic and has a better narrative for an audience, not because it's "better" or "more fair" or "more professional".
That said, in reference to the original example, any poker player who gets butthurt because someone did that is an idiot. Anyone is perfectly free to walk with their winnings... and if they're no good, I'm perfectly fine with taking their money some other time, or with taking someone else's money.
Do they actually televise tournaments with a set amount of hands? A few years ago I went through a phase where I watched a lot of WPT and such, and I'm fairly certain that they don't have fixed number of hands. Players are free to bet within the rules, go all in, etc. They never suddenly stop on hand X and see who has the most money left -- they play all the way until someone has all the chips.
Then the game is edited to fit within the time they have to show it on TV, showing the most exciting hands and cutting out all the hands where everyone just folds all around.

Just wrapped up a Trader/Feodum game where I three piled the Silvers, Feoda and Duchies.
I suppose in reality it was just a boring duchy rush game wherein I proved to be a exploitative, dishonorable wretch for ending it in 3 piles while the other fool tried to run out the Provinces, because, you know it wasn't at all the tactically intelligent strategy.
Nope I applaud your strategy ANY alternate VP strategy SHOULD aim to three pile the game before all of the provinces have been pulled.. Please remember, everyone I'm referring to premature three piles, such as grabbing an estate and three-piling for the win on your turn
I finished a colony / feodum game with traders, bandit camp, journeyman and festival... My deck was so well executed that I was able to pull 3 of the 8 colonies along with 6 feodums that were worth 8 VP each
The problem with this argument is that "premature" is arbitrarily defined. 3-pile endings don't just come out of nowhere. Part of the strategy of Dominion is watching the piles as they run low and paying attention to your opponent's deck. How much buying power do they have? How many +Buys can they accumulate in a single turn? There are game recaps in the forum from top players where they talk about how these things factored into their decisions.
If a player is able to buy an Estate and then 3-pile, it's because they built a powerful deck with lots of buying power and +Buy. Or the piles got low and you didn't react properly.
Yes, sometimes it does come down to luck, but that can happen outside of the 3-pile rule too. Oh, I just so happened to connect Tournament+Province first. Hey, my Sea Hag flipped yours three times in a row. Wow, I just so happened to have that single Curse in my hand every time you played Mountebank, and you somehow never managed to block my MB despite you having 6 Curses in your deck.
In the long run though, skill trumps all of these luck-driven victories. A player who always goes for Treasure Map might scrape a fluke win over Stef or WW once in a while, but they'll get crushed the vast majority of the time.
Dominion is a game of both luck and skill. Part of that skill is in making your own luck, and making
the most of your luck. There is no dishonour in creating a deck that can threaten the 3-pile and ending the game on a win when you are able.