I mean, they kind of did. The Vikings didn’t so much invade with force as they emigrated en masse and brought their culture with them, eventually overtaking the native Britons as the dominant cultural force in certain regions, particularly what would become the Danelaw. It was a slow cultural shift over decades and centuries, not a bloodbath over a few years.

There was absolutely fighting, because that’s what you did back then, but no archaeological evidence has ever been found to support the narrative that the Vikings showed up and took the island by force, raping and pillaging as they went. The few written accounts of this are likely specific isolated incidents, embellished by the writers. They just kind of showed up and intermingled with the post-Roman Britons and had families.

Also, the Celts did the same thing earlier in history and imported their culture from other parts of the continent onto the Isles, supplanting the cultures that existed in Neolithic and Bronze Age Britain, such as the Beaker culture. There is no reason to treat the Celts as anything nearing “native” or “original” Britons. They were yet another in a long history of people who migrated to the part of the European continent that would eventually become the British Isles, bringing their culture with them. The Celts are a relatively recent culture to show up in that area of Europe and treating them any different than the Vikings is, frankly, ridiculous.

History is more a series of slow regional cultural shifts than it is a series of changes by force of violence through invasion. Those events happened, but they’re not as impactful to the whole of history and the changing of eras as pop culture and pop-history would have you believe.