I get you, but it doesn't clearly indicate the angle in the middle at the base as much as it suggestively waggles its eyebrows towards 90⁰, it could just as easily be 89.9999999999999⁰, although upon zooming in, you can see the line does shift one pixel over on its way up. You simply can't trust any of the angles as 90⁰ unless it's got the ∟ symbol (that's the official unicode) or you've measured them yourself, and with that one pixel off-set, it's decidedly not 90⁰. That's why you have to do the math.

Geometry diagrams in math problems should never be assumed to be to scale

Another way to look at it is that it is simply a representation of an object. We don't need to visualize the angles, as the values to the other asks are given. We just need the geometry of the object represented so we can calculate the value of the unlabeled angle. Given that the geometry of the objects is represented as triangles, we can infer that all sides are straight lines, regardless of the type of space the object occupies.

Actually, it might be a 90 degree angle, but the shape on the left might be a quadrilateral instead of a triangle.

Even if they are straight lines, if that's a 2d projection of something on a non-flat 3d surface, it can also change the way the angles fit together.

If these aren't straight lines, drag has no idea what the answer is and thinks it might be impossible to tell.