Iterate through a C++ Vector using a ‘for’ loop

The reason why you don’t see such practice is quite subjective and cannot have a definite answer, because I have seen many of the code which uses your mentioned way rather than iterator style code.
Following can be reasons of people not considering vector.size() way of looping:

Being paranoid about calling size() every time in the loop
condition. However either it’s a non-issue or it can be trivially
fixed
Preferring std::for_each() over the for loop itself
Later changing the container from std::vector to other one (e.g.
map, list) will also demand the change of the looping mechanism,
because not every container support size() style of looping

C++11 provides a good facility to move through the containers. That is called “range based for loop” (or “enhanced for loop” in Java).
With little code you can traverse through the full (mandatory!) std::vector:
vector vi;
…
for(int i : vi)
cout << "i = " << i << endl; Is there any reason I don't see this in C++? Is it bad practice? No. It is not a bad practice, but the following approach renders your code certain flexibility. Usually, pre-C++11 the code for iterating over container elements uses iterators, something like: std::vector::iterator it = vector.begin();

This is because it makes the code more flexible.
All standard library containers support and provide iterators. If at a later point of development you need to switch to another container, then this code does not need to be changed.
Note: Writing code which works with every possible standard library container is not as easy as it might seem to be.