why not ALWAYS use ArrayLists in Java?

If you need a collection of primitives, then an array may well be the best tool for the job. Boxing is a comparatively expensive operation. For a collection (not including maps) of primitives that will be used as primitives, I almost always use an array to avoid repeated boxing and unboxing.

I rarely worry about the performance difference between an array and an ArrayList, however. If a List will provide better, cleaner, more maintainable code, then I will always use a List (or Collection or Set, etc, as appropriate, but your question was about ArrayList) unless there is some compelling reason not to. Performance is rarely that compelling reason.

Using Collections almost always results in better code, in part because arrays don't play nice with generics, as Johannes Weiß already pointed out in a comment, but also because of so many other reasons:

• Collections have a very rich API and a large variety of implementations that can (in most cases) be trivially swapped in and out for each other
• A Collection can be trivially converted to an array, if occasional use of an array version is useful
• Many Collections grow more gracefully than an array grows, which can be a performance concern
• Collections work very well with generics, arrays fairly badly
• As TofuBeer pointed out, array covariance is strange and can act in unexected ways that no object will act in. Collections handle covariance in expected ways.
• arrays need to be manually sized to their task, and if an array is not full you need to keep track of that yourself. If an array needs to be resized, you have to do that yourself.

All of this together, I rarely use arrays and only a little more often use an ArrayList. However, I do use Lists very often (or just Collection or Set). My most frequent use of arrays is when the item being stored is a primitive and will be inserted and accessed and used as a primitive. If boxing and unboxing every become so fast that it becomes a trivial consideration, I may revisit this decision, but it is more convenient to work with something, to store it, in the form in which it is always referenced. (That is, 'int' instead of 'Integer'.)