Because the UK wants them. They can stop the boats at any time. They can stop feeding and housing them at any time. They come because the UK government and NGOs provide them with a comfortable life.
Boris Johnson did Brexit "to stop the immigrants" ... and opened the floodgates. The very opposite of what his campaign promised. The very opposite of WHY the British public wanted Brexit.
This is why the problem hasn't been fixed. You can't say "just do X" when X isn't legal. Do you really think nobody's thought of this? That the government actually likes being in this position of the past decade or so?
But that aside: why come to the UK?
The UK chooses to interpret and enforce them as they do. Other european countries are within ECHR limits and deal with it entirely differently.
The UK elites no longer have the EU to blame for their policies, so they speak about ECHR. On the continent people barely know ECHR exists.
So don’t be so sure about “can’t” versus “won’t”.