Yeah laptops and desktops are form factors optimized for productivity *for most people. The touch tablet form factor is fundamentally at a disadvantage. They’re better for portability, but bad for text entry, which is necessary for productivity for most people. Until we invent something better than keyboards, the only way to make tablets good at text entry is by physically turning them into a laptop, but it’s always going to be a compromised one. And touch is more intuitive and fun, but in a touch UI with larger targets, menu items have to be buried deeper, which requires more time to access. And touch requires more movement and holding up one’s arm which adds up to significant extra energy spent throughout a workday. One can add a mouse/trackpad and have a dual UI (no small feat), but again, the only way to make tablets better for productivity is by turning them into compromised laptops, and, if pushed far enough, it will eventually compromise the tablets as well.
So rather than long periods of productivity, tablets are optimized for shorter bursts of interaction, which makes them ideal side devices and consumption devices. Like you said, the software reflects that. Apple has been pushing iOS/iPadOS further, allowing more people to use iPads as their main productivity device. And Apple will always continue to add more functionality, but likely on an increasingly smaller scale, like the yearly updates of macOS. iPadOS seems to be entering maturity.
I’d also like to mention, though the main and obvious benefit to making the iPad more Mac-like seems to be that for those who use both devices, they get to have/carry just one device—but even that isn’t completely true. The Magic Keyboard is in essence a device itself, bulky and heavy, except with the major disadvantage of being a paperweight without an iPad attached. An ultrabook laptop and a light tablet (if one needs the tablet in addition) together would be only slightly bulkier/heavier, but would have the advantages of being non-compromised devices fully functional without each other, but also more functional together.
*There are cases though where the tablet form factor is necessary for productivity—when needing pen input (though this can sometimes be mitigated by old fashioned pen and paper and scanning), when needing the back camera (though this can sometimes be mitigated by using one’s phone camera and integration like Continuity), when needing to work mobile on one’s feet, or when needing a side computer. But Apple probably sees these as adequately covered by the iPad and other solutions, and/or too niche in the grander scheme of things to make Macs dedicated to them.
Note- I’m really only talking about the fundamental attributes of the tablet form factor, not circumstantial advantages of the iPad (eg. cellular data, better screen).
The reason why ipadOS cannot be a replacement for macOS for everyone is that they have a different set of focuses and priorities. Mixing them would compromise one or both. Apple has already been adding a compromised Mac experience to the iPad in the form of various added features, only because thus far it’s been a tolerable compromised experience, and probably more importantly it hasn’t compromised the iPad experience significantly. But there has to be a point where adding Mac functionality would start to significantly compromise the iPad experience (eg. worse battery life, bulkier device, take up too much storage, slow down). It seems that’s the point that Apple is avoiding.
