I wouldn't say welds are as strong as forgings. Castings, but not forgings. Welds are puddles of molten metal, when they cool the metal gets brittle. Forgings are pressed when hot to alter the formation of crystals in the metal.
Welding is certainly better than brazing of soldering, but it's not at the same level as forging.
Welding is a weak point for a couple of reasons:
First, you have two faces meeting at a sharp angle. Sharp angles concentrate forces at a point, curves distribute forces.
Second, it does melt the two pieces being joined in order to get proper fusion. Doing that means compromising the integrity of the two pieces at the weld.
Welding is a lot better than brazing, which is a lot better than fasteners, but that doesn't mean it makes perfect joints. Bending operations with large radii will usually result in a better joint than welding, it's just not always possible, and certainly not always cost effective.