One reason: When you do need help for builders, you really need it. Help for the Builder's Hut, is kept/stored and does not expire until you use it; but the help for culture burns up /expires. And, as you point out, there are plenty of culture buildings out there to help. As the BH is helped, then folks are forced to choose another structure to help (usually culture). The Builder' Hut is a unique building and is harder to find on the browser version. True, it is much easier to help (in all forms) on the mobile version of the game. But, not all players use the mobile version, yet.
Bottom line: many folks prefer help to the Builder's Hut, as a priority, and then prefer help for culture buildings as a secondary choice. Because this is what they prefer, this is also how they help (in many cases).
As you build relationships with your neighbors, you will hopefully find plenty of folks to help (in all forms).