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This is for a couple reasons:

1. Your odom frame doesn't have feedback going into it. All odom does is integrate your velocities to compute a pose relative to where you started from. I'm not super familiar with cartographer, but I suspect that it uses the map and laser scan data to feedback into your odom pose. Ie, odom provides a rough guess as to where the robot is, and cartographer refines that guess.
2. Sensors are complex (citation needed).They have biases, random walks, scale factors, alignment issues, not to mention that all those change depending on temperature. When you integrate a linear velocity for example, you're also going to be integrating the error of your sensor.

Bottom line is that your odom frame drifting is quite expected. If your robot position is accurate enough for you, then you shouldn't have any issues with odom. If you're concerned about it drifting, then you'll need to do some more advanced filtering of your sensor data or get better sensors.