Just so it's explicit, the full list of default point sizes for type in OS X is as follows:
9, 10, 11, 12, 13, 14, 18, 24, 36, 48, 64, 72, 96, 144, 288
There are two competing historical reasons why this set was probably chosen.
Before System 7 (released in 1997), all fonts on the Mac were bitmap fonts, rather than TrueType outline fonts (chiefly because the latter didn't yet exist).
That meant each font had to be carefully drawn as a specific combination of pixels at each supported point size, and therefore point sizes other than those specifically defined in the system could not be used.
In the original MacWrite, then, the available font sizes were 9, 10, 12, 14, 18 and 24 point (as shown below):
Image from TechCrunch
The original Mac's display was 72 pixels per inch, so 9pt type was rendered exactly 9 pixels tall. Any sizes below 9pt couldn't be reasonably supported on the low-resolution display:
Image from ScotConnect
In the physical realm before desktop publishing or phototypesetting, type also had to be hand-crafted at each specific size that the typographer wanted to support, and it too had its own list of traditional movable type font sizes:
- 3 pt: Excelsior (US), Minikin (Brit.)
- 4 pt: Brilliant
- 4.5 pt: Diamond
- 5 pt: Pearl
- 5½ pt: Agate (US), Ruby (Brit.)
- 6 pt: Nonpareille
- 6½ pt: Minionette (US), Emerald (Brit.)
- 7 pt: Minion
- 8 pt: Brevier, Petit or small text
- 9 pt: Bourgeois or Galliard
- 10 pt: Long Primer, Corpus or Garamond (c.f. Garamond)
- 11 pt: Small Pica or Philosophy
- 12 pt: Pica
- 14 pt: English, Mittel or Augustin
- 16 pt: Columbian (US), Two-line Brevier (Brit.)
- 18 pt: Great Primer
- 20 pt: Paragon
- 21 pt: Double Small Pica
- 22 pt: Double Small Pica (US), Double Pica (Brit.)
- 24 pt: Double Pica (US) Two-line Pica (Brit.)
- 28 pt: Double English (US), Two-line English (Brit.)
- 30 pt: Five-line Nonpareil (US)
- 32 pt: Four-line Brevier (US)
- 36 pt: Double Great Primer (US), Two-line Great Primer (Brit.)
- 44 pt: Meridian (US), Two-line Double Pica (Brit.), or Trafalgar
- 48 pt: Canon or four-line
- 60 pt: Five-line Pica
- 72 pt: inch
In both cases, point sizes are so-named because they're measured in points (which originally varied in size by country, but is now standardised at 1/72 of an inch).
It's probably fairly obvious that it makes sense for these sizes to be multiples of some small text size (so columns of text can share a common baseline grid relatively easily). Depending on the country, that small size would vary, but 12pt (1 pica) was a fairly common size, and so therefore were its multiples, especially in the US where 1 inch was made up of 72pt (or 6 pica), so very large sizes can be referred to in inches (which is why the OS X standard sizes have 144pt, or 1 inch, then 288pt, or 2 inches).
The only sizes in this set that aren't really a traditional size are 13pt and 64pt. Both are multiples of standard sizes, though (13pt is twice as tall as "Minionette" type and
64pt is eight-line Brevier). As far as why they're included in the list by Apple but not in traditional sets, it's probably a safe bet (though I can't find any evidence) that they were considered too similar in size to other similar font sizes.
It's also worth noting the list still starts at 9pt, and probably will at least until Apple rolls out its high-resolution "Retina" displays across its entire range.