Most groups have some policy around members missing meetings. If a member misses too many meetings, then they are asked to leave the board. This is how it is supposed to work.
In real life, some groups have not appointed anyone to monitor attendance at meetings to let the group know when there is an attendance problem. If you have a nominating committee this would be a good job for them, since they will be responsible for finding new board members to replace the ones who can’t make it to meetings.