it's not taught because it's extremely speculative. there's a hodge podge of random evidence, and if you interpret it a specific way it becomes compelling. but...
the phoenician writing - if it's not a hoax - is probably carthaginian. we know that carthage was pretty special in the ancient world, but we have a deficit of their history for the specific reason that the romans wrote them out of history by carrying out a genocide against them and literally tearing down every building in the city to it's foundation, then paving over it. look that up some time - it's dark reading. they were probably more important than egypt or greece, in their era, but they've been wiped out by an act of violence.
we can prove that they had large trading networks all the way around africa (not emulated by a european power until the portugese went out looking to get around the arabs), and north at least as far as ireland. beyond that, it gets sketchy, but it's long been speculated that they did find their way to america. the small amounts of evidence that exist are intriguing, but there's nothing strong enough to rewrite history...
this is in the list of "if they found it, they wouldn't believe it" type theories. but, i think it's very likely. we went through this with the vikings. and it's increasingly clear that the chinese made their own voyages from the other side. give it time.
but to answer the question: why haven't you heard of this? because the evidence isn't powerful enough. at least not yet, anyways.