Aside from being lazy, there is a lack of knowing how to do it and what exactly is required / involved.
People fear the unknown and it is a lot easier for them to just say no straight away instead and end things that to go through all of the trouble and work of informing themselves on what is needed and to actually do it.
It is just a lot easier for them to say 'imposible'.
Since there are already so many people here in Spain who have degrees in 'Filologia Inglesa' and nearby UK and Ireland who are native speakers with EU citizenship it makes even more difficult for them to proove that they could not find any of these first two before hiring anyone non-eu.