Actually they kinda are - the slope of the line suggests that you have to give up about 3.5 atk to gain 1 hp, which is why high hp event eidolons always look so awful in total power. This is also why I caveat everything with
if you need the hp. I actually often find that trade to be a bargain for AAB builds where the AI does a lot of stupid things so I need a bigger hp pool to offset its stupidity (I AAB AQ5 and most ults - not dark ult, actually I AAB that, too and rely on friends to clean up my mess
) If you don't find the hp useful, do what Slashley does and only look at atk (in which case Tiamat rules, by a pretty big margin.)