>>19
Because:
0.0000000...0001
No matter how many zeroes you add it is still a number with finite length. You're arguing it should be rounded off to 0.
0.49999...
This is 0.4 + SUM[N=2 to infinity]( 0.9/(10*N) ). Do the math, it's 0.5. If you haven't learnt limits or infinite series yet, stay in school.