this is a common misconception, that they are "highly stressed" compared to a Marconi rig. Actually the stresses are pretty similar, just confined to far fewer parts. The mast is loaded in bending rather that compression, which makes it much easier to calculate stresses.

In a Marconi rig, essentially all of the capsizing load is transferred via the cap shroud to the windward chainplate, creating a couple between the mast step and chainplate, about 1/2 the boat's beam. On a freestanding rig, all of the capsizing moment is transferred via the deck partners and heel fittings on the mast. The bury of the mast is typically very similar to 1/2 the boat's beam, therefore the force transferred to the deck partners and heel fitting are similar in magnitude to those in the Marconi mast step and chainplate, except they are shear forces rather than tension/compression. The mast in a freestanding rig is a simple cantilevered beam, vs. the Marconi mast which is an Euler column. Any engineer will tell you they would much rather engineer the former than the latter, even today the latter is difficult to calculate, even using numeric methods such as finite element analysis.

In days gone by, the materials did not exist to build a freestanding mast that was stiff enough (with a reasonable cross section) to work well. But carbon fiber changed all that. It will typically be lighter, and have a lower center of gravity than an equivalent Marconi rig

A freestanding mast has these components necessary to remain standing on any point of sail: mast, heel fitting, deck partners. List the parts in a Marconi rig needed to keep the mast standing and you will fill a couple of pages.