david weaver
I've had a couple of early 1800s saws, and I'm sure some of the 1700s saws were a bit rougher, but there was nothing about the saws that I had that would've led to ugly dovetails.

The explanation must be something else - not having a poor saw necessarily, but lacking the right one in general due to cost, maybe.

What I've seen of the older plates is an issue of consistency - inconsistent hardness within a plate, etc, but even that's not universally true. I've got a saw from 1820 or so with two split nuts, and it's a delight to use, and it's a click or so softer than a modern saw and sharpens beautifully and cleanly. The steel has a really "dry" feel to it (it files off crisply with very little burr). It's a tenon saw, but it would easily cut neat dovetails in skilled hands.

I tend to think sloppy is due to standards. They didn't have bloggers in the old days telling them that commodity furniture needed to have show dovetails. There is some truth to speed being similar for "good" dovetails vs. loose ones, but speed isn't the only thing that occurs with craftsmen. Fatigue, lack of interest, lack of need for perfection, inebriation...

I'm sure that making large carpenters saws was a bit of a challenge before industrialization, but saws large enough to make dovetails would've have been that difficult to make. Everything about the original saws (folded backs, handles, etc) are things that can probably still be done better by hand and with care if the maker is skilled, perhaps with the exception of the saw plate itself, but saw plate quality isn't critical in a backsaw.