The most basic reason points are still around is there's nothing metric that can usefully replace them. Note that word, "usefully." There are a couple of reasons why: (1) as Lauren points out (pun hard to avoid... or resist), 6-12-72 has many more even divisors than decimal, so it's easier to work with, just as 60 is much more practical for angles and time than any kind of decimal equivalent. But (2) in very practical terms, 1/72nd of an inch is a much smaller increment than 1 mm. A 1mm difference in type size is much greater, visually, than a 1 point difference. 10 point type and 11 point type are markedly different sizes, but 6 mm and 7 mm are enormously different. The eye sees incredibly fine differences in typography. One word out of alignment by as little as 1/10th of a point is visible even to the untrained eye, and leaps out at a designer like DayGlo socks at a funeral. So if we tried to shoehorn our type sizes into a decimal strait-jacket, we'd be working with unwieldy fractions all the time.
The no-longer-relevant-in-the-digital-age measurements, like ciceros and agates, are endangered species, hanging on only in protected corners of InDesign and Quark, no longer seen in the wild. Points, like cockroaches, are hardy survivors that will be here long after mankind has rendered himself obsolete.
Like most systems of measurement, points are fundamentally arbitrary, but they have stuck around for centuries because they are so immensely practical. The relatively coarse metric system just isn't a good substitute. For much the same reason, you're not likely to see grams replace carats in the gem industry, either. And for a slightly different reason, the good old British pint will be with us for a long time to come.
