legal issues exist preventing steam from doing this.

if they flag a game as not working on your system but it will work, they just lost a sale and may have legal issues with that games developer
if they flag a game as working but it doesnt work, false advertising and more legal issues

2 computers with the same specs can have different performance.

The best you will have is the minimum / recommended specs, and if that is not enough, then you can use sites like systemrequirementslab, but even that would only be a guidance and not nesarly a matter of fact

The flag is in your eyes.

Liability. To the customer:
Steam would have to offer much more loosened refund options if they overestimated the capabilities of your system setup, be it drivers, interfering software or just misjudged hardware.

Liability. To the publisher:
Steam would be liable for missed sales if they underestimated the capabilities of your system setup, be it outdated hardware lists, misjudging benchmark results or due to interfering software.

The system requirement info fields are form free text fields instead of static fields in which to add or select hardware components from.
Valve would have to have a perfectly written and ranked database in which components are compared and benchmarked, something no vendor, not even specialized sites that benchmark hardware, can provide accurately due to exotic outliers.

No, Valve wouldn't do that