Numerical calculations: Why not use fixed point types for everything?

[Ada novice <shai.lesh@gmx.com>]

Hello,
     I have been thinking that since with floating-point representation the distribution of model numbers gets worse (more and more wider spacing) with large numbers leading to less accurate representation of a number, is it better to just go for fixed-point types e.g. in

type Position is delta 1.0**(-12) range 0.0 to 100_000

and define every variable similarly so that we have an equally spaced distribution of the model numbers between the lower and upper bounds?

Why not just fixed-point types in numerical calculations?

And why not just use decimal fixed point types as in

type Velocity is delta 1E-10 digits 15 

for example if we know what magnitudes of the Velocity are to be expected?

I understand that it is a question of portability as well as the actual delta that would be used would perhaps not be the same for all machines and for all compilers.

Thanks,
YC