The reason is for manufacturing. CPUs have been capable of 64-bit processing for about 15 years, that doesn't mean that they are given enough memory to make running it in 64-bit mode worthwhile.
For a company having to manage both a 64 and 32-bit product line would be a hassle, particularly when the 64-bit version can run in either mode. As a result they can simplify their product offering down and offer a "one size fits all" product that does both. For a company like Intel that means they can produce more of less different types of processor and keep their manufacturing overheads lower.
Then it becomes up to whoever is buying the chips as to how much memory they provide, whether the memory can be upgraded and so on.
Yes, a 64-bit operating system with only 2GB of RAM might be a waste, but a 64-bit processor might actually be a bit faster for certain tasks so the trade-off might be worthwhile to someone who cares about the difference.
This is also partly about CPU binning, where CPUs are made identically but have defects that cause certain parts to fail test or are disabled simply to segregate the market. The z3590 for example is a chip based on the same design that is capable of 4GB of RAM.
Functionally the cores might be identical, but one has more parts enabled, can be clocked higher and supports more features. Potentially they could have come from the same manufacturing line on the same day, but been segregated differently when tested.
The result of "binning" is a more efficient production line. You test and separate components and rather than simply discarding defective parts you simply redesignate them with the defective parts disabled.
Binning ties in to the whole "more efficient" production line thing. One manufacturing process, making one thing, and then making use of what comes out of the other side in a more pragmatic fashion.
Having chips that are segregated and yet still "pin compatible" means that a manufacturer can pick and choose what version they populate while still using the same design of motherboard. They could have a 32-bit z3735 version with only 2GB of RAM, and then they could offer a 64-bit version with z3590 with 4GB of RAM with (in theory) the only other change being the (now 64-bit capable) firmware installed on the motherboard.
It may not have worked out that way, but it is a potential option.