Then presumably if the centre moves according to political zeitgeist (The Overton Window) then you struggle to have a defined far-left/far-right.
E.g. ideas that from 1945-1975 were once decidedly centre ground - nationalisation, free education at all levels, universal healthcare, abundant welfare system, strong trade unions, etc....are now viewed as or painted as strongly left-wing, even far-left by some.
The ideas or policies haven't changed - people's perception of them has.
And that's something that's been done deliberately - people haven't just collectively woken up one day and thought, "nationalisation, welfare state, that's way too left-wing".
It's been right-wing think tanks, politicians, newspapers who have pushed into people's minds over time, subconsciously, bit, by bit, by bit.
Sensing the smallest little weakness and digging at it for their own gain to push a right-wing agenda. They started 45 years ago pre-Thatcher and have been at it ever since.
And they get away with it because they are powerful and well funded, because those with power and money want to protect that, and that is what right-wing politics does.