Not being eligibile for citizenship. Not having the right to settle permanently. Not having recourse to the social safety net that your taxation funds. Double taxation (IHS surcharge). It is impactful in a country where citizenship confers actual benefits on you, unlike South Africa in which it doesn't really, as you've pointed out.
I mentioned the short work stay thing before - the only reason to do that is to take something away from the country (be it experience, money, whatever). That's not good for the host country - especially as skills that you obtain/develop will leave with you. Definitely removes anyone looking for any kind of stability or future from the prospective employee pool.
And I 100000% percent agree that the UK should be producing the skills ourselves. But pragmatically, we are not there yet.