Plywood became the standard sloped roof deck in the 70's, prior to this, most properties were built with plank or tongue and groove boards. So why shouldn't shingles be installed on a roof deck without plywood is dead nails. Dead nail is a term we coined to describe when a nail hits between the joints on a deck board and is loose with no pressure. You can easily pull these nails out by hand. Up to about 30 years ago, roofing nails were installed by hand and the installer would know immediately if a nail hit between the boards and was loose. They would then seal the hole on the base sheet or shingle and change the nail line to avoid any more dead nails. Hand nailing roofs is a lost trade as it was replaced by nail guns. Base sheet and shingle installation requires a strict nailing pattern and when using a nail gun, the installer doesn't know of dead nails and keeps on going. Everything looks good but it's left the roof with many tiny holes which are all potential future leaks. When a nail penetrates the roof deck properly, the head of the nails is wedged tightly creating a water tight seal. These dead nails will leak when a roof is left with the base sheet exposed and during a heavy rainfall with the shingles installed as water flows underneath the shingles.
There is no solution to this problem except redecking with plywood. Adding extra layers of protection will not help because when the shingle is installed, any dead nails will still leave a hole in the roof. Installing shingles over shingles, known as a recovery in South Florida, helps but is not 100% as new dead nails are probable. Tile roofs installed with foam will not experience this problem as an underlayment is installed without any exposed nails. Standing seam metal roofs will also perform well with dead nails as the metal panel does the waterproofing.