It's because they are not actually left-wing. They just say they are.
These people tend to have no values or principles that match those of the early Labour, trade union or mutualist movements. They aren't Marxists or Methodists. Most of them support the freedom of movement of capital across Europe and the right for a European company to choose a tax jurisdiction. They have nothing but sneers for the traditional working class.
So exactly how are they left-wing? Because they support the NHS? Support for a nationalised healthcare system doesn't make you left wing. Kuwait has a state funded free-to-access healthcare system; I don't see anyone claiming that makes Kuwaitis a bunch of lefties. 
To my mind, after two decades working in the political field at both national and local level, you have four lots of people scooped together under the label "left wing": proper Marxists, proper Old Labour, Establishment Pod People, and Wanna-Be Revolutionaries (who tend to come from rather privileged backgrounds).
Most of what we see as "left wing" these days are the latter two. The Establishment Pod People are those that hold what they think are "lefty" beliefs but is actually the ideology of the Establishment, which paints itself as "left wing" or "liberal left" but is anything but. Those people have quite rightly figured out that those beliefs are the one they need to espouse and adhere to in order to get ahead in the establishment framework: be that a career in government, academia, the arts, the BBC or the Bank of England.
Interestingly, when it comes to this Establishment "leftism", it's always struck me that it is eerily reminisicent in perspective and attitude to Victorian British imperialist perspectives and attitudes. It's all soaked in this very pseudo-benevolent, paternalistic, arrogant, dismissive, patronising, privileged yet somehow "long suffering" bollocks that insists on classifying people like insects pinned onto green baize. If you disagree with these people, they will a) look at you with pity, as evidently you are never going to be able to do anything for them because you simply won't get into a position of usefulness to their career or b) distance themselves immediately lest you damage their careers and standing with your distasteful opinions.
Then you have the wanna-be-revolutionaries: the "socialist" dude-bros, the SWPers (in their various incarnations), the "anarchists" that have never read a word of Kropotkin ... These are the people who want their very own 1968, are usually rebelling in a subconscious way against their families, and have such a belief in righteousness of their own opinions that to disagree with them is to be a heretic.
However, if you disagree with either Old Labourites or Old Marxists, you will inevitably end up in a pub, with pints of beer stacking up across the table and six hours later, you will be talking about either tractor production, the Paris Commune, or how the Enclosures Act was the beginning of the slavery of the honest working man.
Again, this breakdown sheds some light on how support for immigration and abortion became somehow "left-wing" in the West (read: part of the establishment ideology), when freedom of movement, even within national boundaries, was heavily proscribed under Communism/Socialism in the East. Again, the most well-known country that pursued a pro-life agenda to horrific consequence was a Soviet satellite.
When you pull back from this even further, the real issue is that we live in a hyper-capitalist (even anarcho-capitalist when it comes to finance) economic environment, which is completely supported by establishment ideology, with supporting social and cultural positions sold to us as "left wing". It's a gross deception really.
I can do a similar breakdown of the "right wing", I hasten to add. The alt.right is fascinating for just how un-right-wing it really is, and the icons of the "intellectual dark web" are perplexing in how they identify all the problems of the world as the fault of "marxists" and "postmodernists", which is their mislabelling of the Establishment Pod People.