The bug

Statistics track various kinds of interactions with user interfaces, involving both blocks and entities. Below is a list with every one of those cases, as of 19w11b:

• Talked to villagers
• Traded with villagers
• Shulker boxes opened
• Chests opened
• Trapped chests triggered
• Ender chests opened
• Barrels opened
• Dispensers searched
• Droppers searched
• Hoppers searched
• Interactions with crafting table
• Interactions with brewing stand
• Interactions with beacon
• Interactions with furnace
• Interactions with blast furnace
• Interactions with lectern
• Interactions with smoker
• Interactions with anvil
• Interactions with smithing table
• Shulker boxes opened

However, the game has some similar interactions that, unlike the ones in the list above, are NOT tracked in the statistics:

• Open own inventory
• Horse
• Enchanting table
• Minecart with chest
• Minecart with hopper
• Anvil _{added in 1.15.2-pre2}
• Grindstone _{added in 1.14-pre2}
• Stonecutter _{added in 1.14-pre2}
• Cartography table _{added in 1.14-pre2}
• Loom _{added in 1.14-pre2}
• Stonecutter _{added in 1.14-pre2}
• Fletching table  _{functionality upcoming}
• Smithing table  _{functionality upcoming}
• Command blocks  _{not available in Survival}
• Structure blocks  _{not available in Survival}

I can see no particular reason for these interactions to not be tracked. So, my guess is that whoever added them to the statistics just missed the cases above.