Lots of Western media isn't blocked, CNN isn't blocked
What was blocked recently was BBC in response to CGTN getting blocked first in the UK
Facebook, Twitter, and YouTube were blocked because they refused to take down terrorist recruitment pages and accounts. Remember 2011 ISIS Twitter? Like that. Those services were available before
Additionally, the idea of Internet sovereignty is important to a healthy Web industry in any country. Without the firewall, companies like WeChat and Douyin/TikTok could not have developed
Furthermore, getting around the firewall is trivially easy. OpenVPN on port 443 does the trick, I've set one up for a friend before. Anyone who wants to get a VPN account and browse the wider Internet is freely able to, and many do this
Finally, censorship has a purpose, and that's public safety first and foremost. China doesn't have large conspiracy cults like QAnon or antivaxxers, because rumor-mongering gets reported and dealt with seriously. It's like shouting "fire" in a crowded theater, stopping it is censorship but also perfectly legal. They don't have graveyards full of children or measles, mumps, or rubella making a comeback because of this