Not an expert but stamped parts are pretty weak from some angles, but perhaps you meant cold forged which is how aluminum bicycle cranks and some other parts are made - pressed using 100,000 pounds (or some big number depending on material) of pressure, this of course only works for mostly solid items. There is a huge cost in making a cold forge and a limited number of times a mold can be used before a new mold has to be made due to wear and tear. Frame quantities in the exact same shape are not that big, though, manufacturers do seem to come up with new models every few years. Making a frame from a solid piece of metal would make it very heavy increasing the material costs substantially, and counterintuitively - weaker than a tube for this application - there is a strength from the shape of a tube that a solid piece of the same metal does not have and if my memory serves right, the larger the inner radius, the greater the strength (of course you can make a tube so thin the first mishap with a vector in the wrong direction crushes it, but that's a different problem). I recall there being a magnesium frame that was a one piece thing a long time ago.

Lots of aluminum bike frames are already made from hydroformed tubes. Anything that looks like it wasn't just made from pipes, is molded. The question is whether making a frame from a smaller number of molded parts is cost effective. The different parts have to be of different thicknesses, and there's just the geometry puzzle of how to do it without ending up with a big seam that has to be welded anyway.

Early Schwinn frames included parts (head tube, bottom bracket) that were formed from sheet metal. One thing about making tubes from tubing is that there's very little waste.