Firstly, the hardware cost. It's expensive and slow, but for a business it's a justifiable expense. The real cost is in the ongoing hosting and maintenance. Keeping Mac minis or whatever in a datacentre is a costly waste of space. There's zero remote management, and any failure means a trip to visit the system along with a monitor to hook it up to. Then there's the ongoing system administration cost, keeping it updated, keeping the development tools and build dependencies current. This is an ongoing expense.

Typically, if you need to keep clean environments you need to virtualise, and this means using VMware Fusion or similar. It makes a slow Mac mini system even slower. But if you need to support multiple MacOS versions, or multiple environments, what other choice do you have? It's not like there's a containerisation system for MacOS.

Every other platform can be run virtualised on big metal, like VMware and OpenStack clusters. Linux, BSD, Windows, everything we need to care about. MacOS is a special snowflake exception. This nonconformity adds costs.

As for renting. It's bloody expensive for very little in return. That entire market segment exists solely due to how terrible the hardware and software licensing options are. And the hardware and software procurement and management pain is reflected in the uncompetitive pricing. If Apple offered a developers-only generic VM licence for MacOS this market would vanish overnight.

As a company, adding an additional different testing infrastructure (rent a macmini farm) with the additional pipeline costs (setup, build pipeline etc) as well as the additional opex, again, for a small potential market, is not economical.