Clay is a fundamental resource of the game, you need it for buildings and crafting more advanced resources (bricks), a bit like wood or stone.
Problem is, it happen quite often to not have access to a sufficient source or any at all. With another resources like sand and glass, it could work, because you only need it for 2 luxury products, not as a base materials for every buildings and upgrade.
The trade is answer isn’t valid too, trade is far too much unreliable in terms of price and quantity offered (and you need clay and bricks to access more traders to have chance to get more clay…)
I don’t really see what it brings to the game to have to deal with the scarcity of such a basic resource, it just slow the game down, where you just do nothing, waiting for a trader to come by and hopefuly sale you some clay or bricks. (Yes, I know you can order resource for a premium, but the price is just abysmal and it’s not a valid stratgey to rely on this until you reach T4)
Even a scarse wooden map dosen’t pose such a problem as you can plant trees to “fix” the problem. Clay-poor map are just a pain in the ass, and you can just reroll.
Thats why you have scouts at the start to scan the area.
Whenever i start a town, i always check for nearby willow, herbs, clay and at least 2 deer spawns.
But even if you have next to no clay (had a run where the only nearby clay was just a couple hundred) you can certainly deal with clay via trading, to the point where you can expand to far reaches for sufficient clay.
Having once forgotten to check for willow, that is now also a deal breaker for me, TBH.
I’ve done the “you must import clay” thing, and it’s annoying, but it’s not as bad as needing to manually micro basket manufacturing, partly because there are many more merchants who sell it. (Oh, also because clay doesn’t expire, unlike willow)