Preparing for exploration and preparing for combat encounters both take time and skill. Since 3E, play culture has emphasized the latter, so most GMs have put effort into crafting encounters. If you came up with 3E, you probably feel creating and running combat encounters is a core task of being a DM, while creating engaging exploration content is a bonus 'nice to have.' However, it's not inherently more difficult to prepare exploration than combat encounters. It's just different.