Yes, it can be sad when an art team leaves. I am usually fortunate enough to get a good replacement, but when someone great moves on, it can be very sad. It can take a while to get a groove going with an artist, I always hate to break that up.
There are a ton of reasons why it happens; the artist will go do a book they have wanted to do, they get called for a special project, they get ill, they can’t keep up, they don’t get along with writer or the editorial team, they want a new challenge, there are tons of reasons.
I am lucky enough that the great majority of the artists I have worked with still like working with me, that makes me happy. Sometimes you create serious professional bonds that last forever.
I had one artist leave for religious reasons, basically, but in general, it’s not a personal thing.