The first and sencod note is one step, so in Eb major, do you think that Eb is the first note while send note is Fb or E# makes sense ? Why is F?
In any key, the scale representing those notes (in order) need to each have a separate letter name. This is so that when written on the stave, each note has its own line or space. So, there's an E,F,G,A,B,C and D. One of each. In Eb, there are 3 flats, due to the spacing of each note - 1>2 tone, 2>3 tone, 3>4 semitone,4>5 tone, etc. So E, A and B need to be Eb Ab and Bb.
If the second note was Fb, as you suggest, it would only be a semitone from the root - it would be wrong. It can't be called E# (enharmonic to F), as there's already an E of some sort, and we try not to mix # and b anyway.
All major scales follow the following pattern of Whole steps (W) and Half steps (H): W W H W W W H.
All letters must be represented in alphabetical order (A, B, C, D, E, F, G, etc.)
So and Eb scale would be:
Eb F G Ab Bb C D Eb do re mi fa sol la ti do I ii iii IV V vi vii* I
This is why Ab is called Ab, not G#. You can not have the same letter twice and all letters must be represented. And why F is called F, not E#.
Hope this helps.