There's an, alas not updated since a long time,
RPG Maker emulator-like made with Ren'Py. So I have to disagree with your assumption that it can't be done with Ren'Py. Of course, it need mode coding knowledge than a VN, and Ren'Py isn't necessarily the best choice for this, but it's perfectly possible to implement both the map system and the combat system with Ren'Py.
In fact, regarding the combat system it would always be better with Ren'Py than with RPG Maker since there's no limits in the way you implement it. It can be basic, like in
Damsels and Dungeons, classical, like in
Sakura Dungeon, or more modern, like in
Planet Stronghold 2 ; on a side note, the last title also have a limited map system. I know that there's also a handful of games who implemented it in a RPG Maker way, but no names cross my mind right now.
As for the "secrets in the world", it's something that exist in many Ren'Py games. And unlike with RPG Maker, it can be implemented at any time, not just when you're in a map or free roaming screen.
How many RPG Maker games have you played ? 90% of them use generic assets for the maps and sprites, and among those who have combats, really few are effectively balanced.
And the answer will always be "the one you understand the most".
While I'm not fan of them, a VN made with RPG Maker, by someone who understand what he's doing, will always be better than the same made with Ren'Py by someone who have no clue regarding what he's doing.
By comparing the engines in regard of the way they are mostly used, and not in regard of their capabilities, you also are only talking about game styles.
Take a look at Sakura Dungeon that I linked above. A full RPG game, with old school 3D dungeons, combats, and just a bit of character interaction. Or look at
The DeLuca Family, that mix VN with free roaming, scripted events, and (basic) map exploration.
In regard of capabilities, RPG Maker is limited to old school isometric RPG and basic VN, while Ren'Py offer a way more extended range of possibilities.
It's a bad thing when it add absolutely nothing to the game, what is too often the case in the Western scene. This in the same way that free roaming in Ren'Py games are a bad thing when there's always only one significant location at a give time.
RPG Maker isometric maps make sense only when the maps are effectively used. Therefore when they are populated, either by NPCs, by opponents, or by both. You'll then have a reason to be on the map. Whatever if it's searching the NPC you need to talk too right now, or avoiding combats, the map add something to the game.
But when the maps are empty, and don't have more intent than showing you how far from your actual location is the next place you've to be, then they are just a lame way to extend the time past playing.
Who would play a FPS where you've to pass minutes walking through empty maps for the sole intent of walking ? No one unless it's the purpose of the game.
Instead, when walking you'll encounter people you'll be able to interact with, events that will pop up if you pass near to a given location or near to a given NPC, and game mechanisms like this. Even sandbox games, like GTA or Saints Row, that are fully located on streets, and where you pass most of your time walking, offer you many different activities you can, or not, practice while you move from a location to another. Walking become then a part of the game itself, adding a new dimension not to the game or the game play, but to the game experience and the enjoyment you get from it as player.
Why should it be different with RPG Maker games ?
You don't really know, but it don't prevent you to base all your argumentation on the fact that it's not possible. This when it's perfectly possible ; less easy, but totally possible and in many different way.
Not really more time consuming, no, and not just because most of them use generic assets.
Globally speaking, with RPG Maker, once you've defined the core of your game (maps, opponents, game mechanism), you've near to no more works to do on them. Yes, it's something that need time, but it's something that you've to do only once. RPG Maker is an event based engine. Once you've the core of your game, that is needed right from the starts, you mostly just need to change variables in order to change the state of the game, what will permit to the next events to be triggered. This and obviously writing the story and making the new CGs.
At the opposite, Ren'Py is sequential by default. Having the core of your game will never be enough, and you'll always have to write code. Plus, like the need for the core of the game is not mandatory, too many devs works on it only when they'll finally needs this or that part. What also explain why way more Ren'Py games are abandoned, especially at early stage.
Therefore, saying that RPG Maker games are more time consuming is untrue. They need more preparation time, but once the preparation is done, you'll advance way faster than with a Ren'Py game. This even if the Ren'Py game rely on sprites, and therefore don't need too many CGs.