In the 1980s and following years, a fluke occurred in the electronics space. Largely due to the hobbyist culture at the time, devices which found their way into the workplace and the home exposed users (theoretically) to the entire system. Many of the characteristics which are still fairly common in *desktops today, such as the ability to boot non-vendor approved media, derive their heritage from this era. As laptops took off, they largely upheld these principles, with the regrettable exception of never adopting an open standard for motherboard layouts. On both the hardware and software fronts, users are arguably able to make their own decisions and apply their own changes.
When the concept of the smartphone emerged around 2008, it slowly became clear that this new form factor would one day become the primary device for most computer users. The industry saw a new opportunity to "do it over", from a clean slate. From day one, they were sure to lock the form factor down as much as possible so as to prevent users from having any real freedoms of their own.
Closedness is so widly accepted in the phone space today, that terms like "jailbreaking" have entered normal lexicon. On prior form factors, we just called this "installing your own software". Phone makers were also sure to remove all user freedoms with regards to hardware. Components are firmware locked and nothing gets installed without the vendor explicitly allowing it. This is, of course, if you are even able to access the internals of the device.
A tragedy of this is that the abuses pioneered for phones have begun making their way into the once-free desktop space. CPU vendors now lock down their chips and boards with cryptographically signed firmware, allegedly for security. For whom is the device being secured? Similarly, the freedom to boot any media is being eroded by the industry's meddling in EFI, implementing "secure" boot. Vendors do not actually care whether your system's boot process is resilient toward malicious modification, the real value to them lies in handcuffing the users to vendor's software and further raising the barrier for those who wish to explore outside of the walled garden.