I haven't worked for Samsung in ~7 years, but last I recall they were aggressively pursing all manner of value optimization angles. They also have an existing presence in the US, whereas TSMC is going to be about 5 years out on their US factory. Keeping American workers around to inject ideas into your manufacturing process can be invaluable if you have typically been operating the business like a Borg cube (i.e. relentless standardization throughout, potentially at the cost of more complex value-add proposals).
Happened again and again. In free markets, of course. Even if they may trend to, only controlled markets reach monopoly.
Now the market can arrive to a state of monopoly due to some powerful player, but as long as the startup mechanism is functioning, competition will arrive and disrupt.
1) They dominated their "primary" markets
2) Their primary market was deep, and competing with them was cost prohibitive for any economic entity as in lose the GDP of a small country expensive.
But AT&T - you kidding me?! It was illegal to even bring your own phone device to their network! It was a classical example of government protected monopoly.
A free market will break its own monopolies. A market corrupted by government regulation will need government intervention.
With regards to it being illegal to connect to the phone system with your own device... this was a notion pushed by AT&T's legal, and operations teams. https://en.wikipedia.org/wiki/Carterfone has some useful context on the regulatory decisions which compelled AT&T to allow third party devices on "their" network.
Ultimately large monopolies contort the institutions, governments, and legal systems they are surrounded by. You wouldn't believe that a Trillion dollar corporation would argue for legislation that wasn't in their favor.
I remember when the local telecom monopoly in my town held ski-high Internet prices, regular people started laying up Ethernet cables in the air between high-rises. A new ISP was born that way, who now has completely wiped out the original telecom.
Of course though, there is now a regulation against doing that kind of thing...