While I find your comment informative, points 3 and 4 do not really explain why vendor-neutral tools cannot exist. Point 2 (my best understanding) should not be a breaking issue if documentation and specifications are available (which is covered by point 1). It seems to me that point 5 is part of the thing they have done to prevent alternative tools to come to market. Lastly, point 1 is exactly what begs for disruption.
The manufacturers also treat the exact implementation details of their FPGAs - how exactly all the routing fabric is structured, the fine details of some of the macros, etc - to be their secret sauce that gives them an edge over their competitors. So they're pretty hostile to the idea of documenting anything.
