For which enterprise? If you buy a dyeworks, you'll spend 10.000 denars for 500 denars each week, which is clearly 5% like the lands. Of course if you buy another enterprise, you'll earn less money...
It's true. Buying land typically gives better returns than starting enterprises does; many enterprises go into the red and few can be trusted to consistently stay at or above the 5% ROI mark.
In contrast, land is cheap on a per-plot consideration, allowing you to begin investing earlier; this also means a little bit of bonus cash since money sitting in your pocket waiting to be used does not grow. You don't need to please a town's owner and guildmaster in order to buy land. As azxcvbnm321 has pointed out, if the town passes into unfriendly hands, your land continues to work for you.
Of course, there's the risk of buying too much land (as explained by Aziyk earlier in this thread). The money you make from land also simply sits there unless and until you go pick it up, making enterprises attractive for the fire-and-forget kind of player and for players who like to run long and uninterrupted military campaigns.
All told, the land purchase system is probably "overpowered" and can easily make the financial difficulty of running a large military machine a non-issue. Since it's a feature that's only used by the player, however, it's easy to just ignore it if one does not wish to benefit from it.