Actually, they do (or did, future will tell us).
Apple ][ series (6502) to MAC (68k) compatibility was achieved via an add-on card on some models. 68k to PPC was via an emulation layer. MacOS classic to OS X was done with emulation (my gigabit G4 Powermac did that pretty well actually, and if memory serves it was called CLASSIC, or do it with fat binaries.
Starting with 10.4.11 there were universal binaries that could run on either PPC or x86, or Rosetta could handle them via emulation (up to 10.5.8 IIRC). So they can pull off another architecture change and retain compatibility until vendors come up with updated software.
Kinda fascinating that a Visicalc file could be transfered from a 5.25 floppy from late '70s early 80s all the way to the latest MAC, while Windows running on X64 can't even handle a 16-bit installer without external support...
IMHO what Intel needs to do is ditch anything before P6 in their architecture. What ran on a 66Mhz 486 or 233 Pentium can be done easily with emulation on newer machines.