If you think that the fingers "Can't" be strengthen at all, go to a physical therapist and ask him how many people he has helped regain strength in their hands/fingers.
The actual fingers THEMSELVES do not have any muscles in them, but the fingers are moved by muscles in the palm of the hand and forearm. These muscles, like any muscles, can be strengthened to a certain point.
Your fingers can and do become stronger. However there are some boundaries, for example:
The 4th and 5th fingers will never be as strong or independent as fingers 1, 2, and 3.
Schumann found this out when he tied his fourth finger to the ceiling while he slept. He was trying to increase the strength and mobility of it, but instead he ended up paralyzing it, rendering his fourth finger useless.
Beethoven and most composers of his time thought that the fingers could become of equal strength with the proper training. Later composers, like Chopin, realized that this could not be accomplished.
In a sense, Beethoven, Schumann, Czerny, et al. were trying to make their hands fit the music, while Chopin, Liszt, et al. made the music fit their hands,
Fun fact: Chopin based most of his Etudes around the index (2nd) finger.