Clay pots have been around for a very long time. It is impossible to know for how long exactly. It is logical to say that they have been around long enough to evolve into the perfect shape for their function. Although the dimensions and proportions are variable, the basic design characteristics of the simplest and best engineered clay pots can not be improved on.
Clay pots are circular from above and below for a few reasons. Such a shape is easily formed on a potters’ wheel. It is more structurally sound than a form with flat sides and more corners. The space within is evenly distributed around the vertical center, without more remote corners. Although roots will circle within, there are not so many corners for them to congregate in.
Drainage holes are at the bottoms because that is where water drains to.
Thick rims around the top edges of common clay pots enhance durability where it is most necessary, and also prevent pots of the same size from becoming wedged into each other when stacked. The weight of stacked pots rests firmly and vertically on such rims, rather than being diverted laterally to break wedged pots to pieces.
It would not be possible to stack clay pots if they were not tapered to be wider on top. Of course, they are tapered for a few reasons, just like they are circular and outfitted with rims for a few reasons. Tapered form fits the natural dispersion of the roots of most plants better. More importantly, tapered form facilitates the removal of firm root systems with minimal disruption.
So, after perhaps thousands of years of evolution to achieve the perfect form, who thought it was a good idea to taper pots inward at the top? The lack of a rim is not so important if pots are not so numerous that efficient stacking is a concern. Such pots can not be staked anyway. They do not get reused as much as common clay pots either, so do not need to be so durable.
However, that upper inward taper is a serious problem for plants that mature and develop firm root systems within. Such mature plants can only be removed from such pots only by tearing their root systems apart, or by breaking the pots apart. Such form is only practical for big pots that contain multiple small plants that, individually, do not get big enough to fill the pots.
For example, big urns of bedding plants or mixed perennials function more as planters than as pots. Bedding plants get removed and replaced seasonally, and even if the don’t, they can not get big enough to develop a solid root system that is wider than the inwardly tapered top of a big urn. Likewise, most perennials get removed from such big pots before they get stuck within.
Palms, agaves, yuccas and other more substantial perennials must not be allowed to live within an inwardly tapered urn long enough to develop a firm root system that can not be removed.