Just building on @Preppercorn comment:
"YANBU OP, but you'll be branded a horrible racist for even bringing it up, so nothing gets done year after year even as more and more people slowly realise how bad it is. I hope one day the scales fall on this and people realise it's not racist to be concerned about net migration and question whether there is a net contribution to the economy or not of all"
Unfortunately levels of immigration has now become a topic that cannot be rationally discussed without the accusation of racism being brought up.
It is correct to state that the majority of immigration is legal. From government's short term perspective importing cheap labour (NHS nurses etc) makes sense because it keeps costs down and keeps inflation down. From a company perspective it also makes sense as it keeps costs down and profits up.
A large number of posters are of the view that immigration is essential in order to do the jobs that UK nationals don't want to do. I would counter that legal immigrants are doing the jobs that nationals don't want to do at the salary being offered.
The UK currently has 9.4 million people (22% of the population) between the age of 16 and 64 that are economically inactive. The biggest proportion being long term sick with 7% of the population being economically active for that reason.
Those posters claiming that immigrants are needed to fill the jobs that nationals don't want to do are effectively writing off this 9.4 million as unemployable and throwing them on the scrap heap.
For example the average salary for an Amazon delivery driver is £12 per hour. Now let's say that all legal migration was suddenly stopped. Amazon now have a lot less delivery drivers. At this point they can simply stop doing deliveries or hire nationals at higher wages that they would be willing to work for. This of course leads to higher prices but it does save on taxes being spent on welfare benefits to the economically inactive.
On the long term sick the problem is often that nationals are unable or unwilling to do the jobs at the salaries on offer. As a result of being unemployed they start to develop health problems especially mental illnesses health problems. By getting them into work this saves money on benefits, health care, benefits the individual, benefits society and generates rather than cost tax revenues.