I maybe wrong, but could it do with chainring clearance for the chain stay?

That definitely looks like a possibility.

As said above, it has to do with chainstay clearance. MTBs have their chainstays bow "out" considerably more than a road bike to accommodate wide tires.

Most definitely chainstay clearance. Most of the older bikes were designed back in the front derailleur days, and the largest common inner chainring was 28t. With first-gen SRAM and Shimano 1x, even the most powerful pros like Kulhavy and Schurter raced a 36t, sometimes a 38t (Kulhavy), and even that could be considered overkill (Absalon raced with a 32t usually) - so quite a few frames top out at 34t. Schurter's previous-gen Scott often had scratched chainstays because the clearance on the 36t was too tight.

With the advent of SRAM Eagle, amateurs who used to ride 32t or 34 can suddenly run 36t without a problem (I do, for example) and some pros upsized to 38-40t. That's already road-sized chainrings, and not something older frames were designed for.

P.S: Wouldn't be too worried about being limited for 34t. Unless you're intending to build up a monster-cross rig, for most MTB uses 34t is plenty.

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ZONE3 - We Last Longer

Chain stay clearance is right. I'm looking at my wife's bike, purchased this winter. A 32 on there now, and 34 would be pretty close to the chain stay. That is with a direct mount ring on a SRAM NX drivetrain. You may be able to get away with a different setup with a spider, and move the chain ring to the outer side of the spider - but then you are messing with a chain line.

Even with a 32, I don't feel limited in top speed on her bike on dirt.

Yeah in doing some searches I agree with you both. I know I'm not going to run out of top speed with a 34t or 32t for that matter.

I'm exploring some kind of hybrid gravel grinder. I'm looking for excuses to build something new to ride. I don't want or need a new tri or new road bike. And still completely satisfied with my full suspension MTB.

Gravel is the answer!