Exactly, who says that the Maker can't fit into the Elven Pantheon or that some Dalish might chose to meld the two faiths like Ameridan did?
All things being equal, there wouldn't be a problem. But it's not equal.
First, how many Andrastian humans are interested in fitting the Elven Pantheon to their worship of the Maker? None. How many Andrastian humans choose to meld the two faiths the way they might expect the elves to do? None. So why should the elves do it when humans won't show the same courtesy?
Second, the Andrastian religion runs on the core ideal that only their religion and their god is "true," while all other religions and gods are "fake."
Quoth the Canticles of Light:
"As there is but one world,
One life, one death, there is
But one god, and He is our Maker.
They are sinners, who have given their love
To false gods."
EDIT: And then turn around and tell another people, "And there is not room among your gods for one more?"
There is something inherently unequal in a religion that says, "I don't have to compromise on my beliefs, but you do. I don't have to add any other gods to my list of worship, but you have to add mine." How long before they try to phase the others out? How soon before, "Why don't you add my god to your list of gods?" becomes, "Why don't you worship my god instead of yours?"
Once humans start making room for other gods in their roster of worship, I'll be okay with the Dalish doing so for them.