The majority of maps are not suited for grinding at all and instead simply performing your quests and moving on to the next.
Not all? The majority.
Exceptions are really
West of Klaipedia, East of Klaipedia, Miners Village, Crystal Mines f2.
The first dungeon in Orsha all floors. Tenet Gardens. Nevelett, Novaho Floor 1. Topas, Sitracius. Demon Prison F2. 2nd Demon Prison, 4th DF IIRC, Bee Farm, Roxana kinda. Workers Lodge and Chapel. Mage Tower floors. Main Chamber (but forget the rest of the floors).
Everything else gets pressured significantly once you have notable people in it. Though you can quest in them, once quests are done move on until you reach one of the above areas. Then there’s the extreme maps which are very bare like Sutatius trade route.
The OP has already made his comment on the spawn rate, but in your eagerness to be dismissive you latched solely to trade issues when the spawn discrepancy is understood by most players. It’s also a significant part of the idea of card holding where players funnel into one of the few maps that support multiple mobs while they wait to gain enough exp to avoid all the barren maps. This is obviously not beneficial for most players who are not doing research into the games inability to provide a significant stream of mobs for them. The effect is multiplied by our reduced mission/dungeon cost which creates extra emphasis on maps having sufficient spawn rate to accomodate players. Since most don’t you have significant parts of the community at specific level ranges funneled into 1/2 areas because they lack choice elsewhere.
This is not a minor problem and is frequent within the game.