It's made that way because its much easier and cheaper to machine the barrel from drawn steel tubing, machining it to size and cutting the top lugs, then brazing it into a forged breech and bottom lug. It's as least as strong as a 1 piece because there are fewer things to go wrong than in a 1 piece machined forging. Here's a post from Jim Keenan on the 1911Forum.com:
Barrels for the 1911 type have traditionally been forged to rough shape, then machined to final shape, drilled and rifled. That, as you gather, is a very expensive process. A cheaper method is to make the barrel two piece, using a piece of high quality steel tubing as the barrel itself.
There are two ways to make a 2-piece barrel. One is to make the rear of the barrel as a forged or machined block, with the lower lug as part of it. The block is then drilled out and the barrel tube, with the top lugs cut into it, is inserted into the block. Induction brazed together, that makes a pretty strong setup and has been used by FN in the Browning High Power for many years. I consider that type the equal to a one piece forged barrel.
The other way is to make the barrel from a tube, then weld on the lower lug. This is a much weaker arrangement, and I have seen some with the lug peeling off. I would not own a pistol with that kind of barrel and, if I had one, would immediately replace the barrel with the one-piece type.
I swear by Jupiter Optimus Maximus .... in the army of the consul Lucius Quinctius Cincinnatus and for 10 miles around it I will not steal anything worth more than a sestertius in any one day.