Imagine your country was specifically targeted by malicious propaganda via the internet constantly. You can't use sites hosted by your political rivals, because your citizens will be bombarded with anti-you propaganda.
That's the reality for China, so they blocked the western-backed sites. You can buy laptops with VPNs on them, some of them are supported by the government. But, by restricting easy access to western media, the CPC ensures their citizens won't be flooded with hateful bullshittery on a daily basis. People who decide they want to visit western-backed sites are allowed to, but it's at their discretion, which assumes they understand they'll be faced with shit targetted directly at them.
There's various youtube channels that upload from China, which in itself disproves that google is strictly prohibited. As well as blogs, facebook pages, etc. If you're wondering why you don't often see people from China using western sites that much, which sinophobes try to use as proof that they're not allowed to use the sites, look no further than the sinophobes. People who live outside of the west detest being in our internet spaces, because no matter where you go - you're going to find pages and pages of people shouting hateful filth. They'd much rather use domestically hosted sites, like Weibo and Vkontakte, where they only have to deal with the ideocy of locals and the occasional lost foreigner.
(Also note that western language speaking communities outside of the west tend to gravitate towards imperialist naratives, almost as though the provocation of English outside of the west was on purpose or something.)
really, the "firewall" isn't really that much of a "wall", and is more of a waist high gate. People who want to get over it can, pretty easily. But it keeps predators out, so it does its job.