Because those "floodgates" for the most part have never existed and are just fear inducing rhetoric. Immigration has always been insignificant in terms of the whole population and therefore can not solve systemic problems alone.
Not only is 13 million people not "insignificant" in my book, but clearly the trend is accelerating.
And places become more boring if the have the same migrants as every other place, not less boring.
Since the Anglo-Saxons, we've had numerous intense bursts of migration driven by the first and second Viking periods, the Normans, the Hundred Years War, Black Death, Border Wars, Plantations, Thirty Years War / Wars of the Three Kingdoms / the Huguenots, Plantations, Colonialism, Inclosure, Clearances, Industrial Revolution, Famine, New Colonialism, World Wars, and the Commonwealth.
Yes, the Brexit migration wave was particularly sharp, but the movements in the 1850s-60s were proportionately greater (albeit spread over a dozen years rather than just five).
My understanding is, the "original flood" from India, just like the turkey workforce in Germany, was desperately needed back then, "flooding a dry land" so to say.
Just as in Germany, a significant portion of doctors (30%) in the UK were born in another country. So it seems they (just as Germany) still pretty much rely on foreign workforce which is also why the numbers didn't go down after brexit even with the foreigner unfriendly political climate that caused it.
Such as? It looks like the country is doing much worse now that the border is closed.