It's not that difficult to understand why. Think of the motive... Why has Google and Skype made their solutions available for other platforms? Why hasn't Apple?
Google Voice is free. Like most other Google Service. It collects data about you, your conversations (written), who you call, when, where and from what devices. Google and third parties use this data to serve more relevant ads that are likely to interest you. It is beneficial to penetrate as much of the market as possible, thereby collecting the most data. How do you do this? Give your solution away free to everyone.
Skype has a different business model. They are freemium. They give their service away for free to as many people to drive customer use, with the hope that the free users will eventually become paying users (such as buying credit, use it in their business etc). It is beneficial to make it as accessible as possible, so more users sign up, and eventually convert to paying users. How do you do this? Give your solution away free to everyone.
FaceTime has absolutely no commercial motive. It is a profitless feature Apple has created and put on it's devices to give iOS/Mac users an incredibly easy way to video and audio call over the internet from just an Apple ID. It is there to be another feature of iOS/Mac devices. It is part of a package of software you buy, when you buy an Apple device. What motive does Apple have to spend developer resources and money into software for someone elses ecosystem? Which at the same time erodes their own ecosystems value? None.
Opening FaceTime up to other platforms also creates the possibility of users calling other users that don't actually own any Apple devices. This would then actually be at expense to Apple, because of server use.
Also, on the topic of ringing any phone number... FaceTime is highly encrypted, and is a self-hosted phone call (meaning your device connects directly to the device you're phoning), and the audio/video stream never touches Apple's servers. This is highly secure, working just like iMessage, and is all VoIP. Allowing landline phones to be rang would confuse this message. It would be an insecure, unencrypted, non-VoIP call, that would have to go through Apple's servers and would also charge you. It does not fit the FaceTime branding, or would have the same ease of use.