Because asset polish doesn't matter nearly as much as gameplay: the high-res remake will never make as much as the original.
The most successful game of the year, Far Cry 5, has sold 1/20th as many copies as Minecraft has, a game that is ugly as sin and always has been, but it's fun enough that it doesn't matter.
There's no point in adapting a property if the adaptation won't grow the audience. The Walking Dead was a comic book superstar, selling upwards of 500,000 units per issue when it was picked up by AMC for the TV adaptation. The TV adaptation is now reaching 8 million viewers per episode, and that's considered low.