This would be a good cross-post to Electrical Engineering!
There is more than meets the eye.
Answer First: It is a reasonable assumption in most regions of the world and most applications But it is not a strict requirement of ATX or PC Systems. It is theoretically possible to make an isolated Motherboard and ATX PSU system. But I suspect that it would be difficult to make it compliant with national governing bodies for Electromagnetic Interference (e.g. FCC, EN, CE) and tested by Nationally Recognized Testing Laboratory for Safety (e.g UL, CE) while being competitive and universally compatible. . Isolated systems, which are common in industrial sectors, are not marketed as generic PC parts, but as custom systems or marketed for specialized applications. .
Contentious Claim #1 There is no ATX et. al or System level specification that dictates that Chassis/Earth and Common must be bonded
Intel ATX 12V Supply Design Guide
"ATX Specification" as archived by instructables
In the latter document, the word "Ground" is mentioned very sparsely, without clear guidelines. "Earth" is not mentioned in the document.
For example, in Section 3.3.5, grounding is mentioned in passing in discussion about back panel I/O clearance and paint masking on cases for EMI Compliance. There are useful diagrams of the back panel.
The former document, an Intel design guideline for ATX Supplies.
In section 8.4 mentions "Earth Fault" as a "Recommended" Catastrophic Failure Protection", something that shouldn't happen if the Power Supply fails.
Intel also describes the conditions for Safety Compliance which mentions "Earth"
These are not the authoritative documents, but the lack of any specific reference to earthing or grounding leads me to claim #2
Contentious Claim #2: Bonding Chassis to Earth and shielding everything is the most expeditious way to comply with electromagnetic interference requirements (EMI)
What may not be transparent to you, but the PC world , as an industry, is specified such that if you build a system following the MFG instructions and you use certified parts your final system will be compliant with the regulations in your region.
This is why every ATX Motherboard must come with a back I/O face plate.
This specification is easy to describe but difficult to define.
One aspect of EMI testing, involves radiated emissions, you point an antenna at the system in a shielded room.
source: science direct
Another key requirement is Electrostatic Discharge, where a high voltage spike (1-4KV) is applied to various pins and metal parts of your device/system and it is expected to not respond or recover on its own.
As a whole, EMI testing is similar across the world. It is not surprising that a global product like PC and PC components has settled on a common denominator that keeps everything compatible and keeps the regulators happy. PC's are very (radiofrequency-wise) noisy constructions, and following this pattern keeps everything quiet in the neighborhood.
The last aspect is how to reconcile this with safety
Contentious Claim #3 : If you must have chassis bonding to facilitate emissions, and you also must provide a path to electrostatic discharge to ground, then a non-isolated supply Class I is the most obvious standard.
Class I supplies require a PE connection, which was already (per claim #2) required to be electrostatic compliant and must be connected to ground for shielding compliance, and thus you see this little symbol on power supplies and systems.
The basic difference is that Class I supplies only have a single layer of isolation between the Power circuitry and the chassis, and thus a protective earth path is required for safety, and all metal sufaces must be bonded to protective earth. Subsequent, because it must also be bonded to EMI shields, and the shields are bonded to ground, this is where the connection between Protective earth and Ground is again reinforced, This is then reflected in the test by the Nationally Recognized Testing Laboratory which will specify (on a per-region/country basis) the requirements for the relevant label (e.g. UL in NA, CE in Europe, CCC in China)
Comparison of Class I and Class II from XPPower https://www.xppower.com/resources/blog/iec-protection-classes-for-power-supplies
Interestingly, the Intel design guideline does mention a power supply feature in section 3.3.2 about the Power On Pin, but does not mention isolation as a requirement for any other pins.
A search of "Class II ATX supplies" (with roman numerals, not class 2) shows much fewer options in the standard locations.
This leads to the conclusion
Contentious Conclusion Universal Compliance with EMI requires using the metal chassis for shielding and safety regulations reinforce the requirement that the chassis must be bonded to earth. The combination leads to the general assumption that the Protective Earth, Chassis/Metal Case, and system"Ground" or "Common" must be isopotential, in most compliant situations.