I'd guess it depends on how narrow or wide you define the "desirable" group. Research from dating sites often conclude that women select for the top 20% of men whilst largely ignoring everybody else.
I imagine such a "top" man to be handsome, sociable, strong, economically successful or at least stable, funny, and whatever other quality women select for.
The chance for such men to go unnoticed and unclaimed until they are 30-40 y/o has to be close to zero. Almost all female attention is channeled to them, and they're unlikely to be passive themselves. The market should be efficient here. Do you know of any hot successful men that are single due to not being discovered?
If you widen the selection criteria to the best 50% of men, you're likely right that some of them in their 30s go "undiscovered". Yet it's questionable whether this is so in large numbers as time still matters:
https://www.pewresearch.org/social-trends/2020/08/20/a-profi...
In the age group of 30-49, 27% of men are single. Half of them want to be single and are not on the market, so that leaves 13.5% of remaining "supply".
So a single woman in this same age group looking for a man finds 86.5% of men unavailable or not interested. Did the market perfectly select the "best" 86.5% already? No, human affairs are complex, but we also should not look away from basic dynamics. Nature finds a way, whether we like it or not.
In my country (Netherlands) already from age 20, only 20% of men is single. It's unlikely that all remain with this same partner for life, but this is to point out that selection start early, and almost all "supply" is immediately taken. I don't know if the market is efficient, but it sure is fast. When trying to find your first partner in your 30s, you're a decade too late if you have high standards. In your 40s, most people have a family with kids that might be up to 10 years old, so "late" doesn't even begin to express it.