The trouble with considering Einstein (or any other intellectual genius) a philosopher (or an expert in any field in which they did not establish themselves) is that it's difficult to separate their reputation from their accomplishments. Strictly speaking, Einstein was both a lover of knowledge (the literal meaning of "philosopher") and a deep, effective abstract thinker. Naively, one might imagine that a genius in one field will be at least a standout in any other field that requires similar mental ability.
Sadly, that is not so. Consider, for instance, the theology of Isaac Newton. Newton must be seen as the equivalent of Einstein when it comes to physics. Swap their places chronologically, and it's entirely possible they would have been capable of making each other's discoveries in physics and in mathematics. Like Einstein, Newton avidly thought of the "big picture". Most people would be severely tempted to accept his thought on any topic up-to and including philosophy.
But Newton's theological ideas were mostly unknown in his own time (because of the threat of being tried for heresy) and as they are unearthed and examined with modern eyes, seem bizarre and unscientific. While it makes sense that a man so instrumental in calculating the past and future position of heavenly bodies would be interested in calculating the times of the start and end of the world according to arcane prophesies, almost nobody (Christian or not) would agree with any of his conclusions. In a sense, his approach to teasing out the mysteries of physics and solving the problems of calculus proved wholly inappropriate to teasing out the mysteries of the Bible and solving the problems of prophesy.
I suspect that in time, Einstein will join the ranks of scientists who were considered out of their depths in philosophy and other fields. The list would include Pythagoras (philosopher, mathematician, and founder of the religious movement called Pythagoreanism), Johannes Kepler (mathematician, astronomer and astrologer), Tycho Brahe (astronomer and alchemist), and of course Isaac Newton (physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian).1 That's not a slight on these great thinkers, but a warning that we ought not take all their ideas as words from on high.
On a more personal and anecdotal note: scientists seem to dedicate themselves to intense study within their field and are often somewhat ordinary thinkers in unrelated fields. It may be that the technological and mathematical demands of modern science make becoming a contributor in multiple fields more difficult than in the past. For reference, I learned the Pythagorean theorem in 6th grade, much of Newton's contribution to physics and math in high school, and struggled through Einstein's work in college. Considering the equivalent progress required to be an expert philosopher (and student of philosophy), it seems less likely than ever before that we will discover true polymaths in this era.
- All descriptions taken from each thinker's respective Wikipedia page.