Why do you think that you as a mainstream teacher should be trained in "more able teaching" but not SEN?
I think the breadth of the term 'SEN' is the issue here.
Let's think of a Maths teacher. They are trained, rightly, to teach those who find Maths hard, those who can be taught to do Maths well, and those who are gifted at Maths - essentially a linear progression along the same spectrum.
However, any child at any point in that spectrum could have SEN. Yes, there is the strictly maths=-related SEN of dyscalculia, which I would expect a well-trained Maths teacher to be trained in.
However, is it reasonable to also expect that Maths teacher to also be comprehensively trained in every type of SEN they could ever encounter, ranging from PMLD to severe autism, taking into account all levels of physical, sight-related, hearing-related, attention-related, processing-related, everything that goes under the heading of 'dyslexia' from low working memory to tracking issues and beyond, etc etc needs?
It is in fact more viable to train a primary school teacher, because of the relatively limited numbers of pupils with SEN they encounter each day in any given year - so one can imagine a process by which a primary school teacher with, say, between 10% and 40% SEN children in their class [I have never taught a class with over 40% or less than 10%, though I am sure that they exist] could receive specific training in the specific conditions their following year's class contains.
It would be much harder for a secondary school, in which every member of staff teaching every subject taken by a specific pupil would need training, and each member of staff could encounter many tens of SEN pupils per week.