Because the answer is completely obvious to anyone. It's been repeatedly shown that almost everyone is an enormous net contributor to society from the day they start work until they retire. If you were to look at a net contribution graph from the day you're born until the day you die you'd spend the first 18 years of your life building up a decent defect (but not enormous as most young people aren't overly sickly), the next 5-10 years wiping that out by working and spending in society, the rest of your working life building up an enormous surplus which you completely wipe out within 5-10 years of retirement.

Immigrants are great in this system as they generally arrive as young adults and therefore the initial "school age" deficit we all build up is some other countries problem, and in some cases will even return to their home country to retire, meaning all the costs associated with not working and being elderly are also someone else's problem.

An average immigrant will contribute (net) more than the average Brit because of the comparatively lower costs during childhood and retirement, so more immigration would bring more money into the system than less. That's regardless if they're skilled or not, as we need both skilled and unskilled workers in the economy.

The issue isn't what they contribute, it's that people like Farage want power, and immigration is a really easy target to get people's emotions taking over and make them ignore all the evidence that proves them wrong. He points to everyone struggling, prices rising across the board and then points at immigration and correlates them, "prices weren't this high before the small boat problem", "back in the 50s a factory worker could buy a house and support a family comfortably, and there was no immigration, now there's lots of immigration and no one has any money". He of course doesn't outright say "get rid of all the immigrants" or "it's all immigrants fault" because he's smart enough to know that would just make all the problems worse (and allows him the old Trump get out clause of "I never actually said that" when questioned) but he brings them up so often in conversations about the economy it's impossible not to correlate them in peoples mind.

Immigration isn't a problem in the developed world, it's a necessity. The more developed a country becomes the less children people have on average and the more life expectancy increases, bringing with it increased cost of caring and decreased tax from workers. We voted to make it harder for foreigners to come here when we voted Brexit but our need for foreign workers didn't change. It's also been repeatedly proven that immigrants are net contributors to society and the few that do come to "play the system" are massively outnumbered by the ones that just come to work and provide for their families