Why don't globular clusters flatten with a galactic disc? Globular clusters lie in the galactic halo, outside of the disc. However, galaxies are more or less a collection of material and objects — why is it, then, that most stars form a plane due to the angular momentum, but some patches of stars do not?

In fact, globular clusters often contain some of the earliest stars formed in a galaxy, right? How come they do not flatten, yet a lot of the older stars do?
Globular clusters formed whilst the gas of the proto Milky Way was still approximately spherically distributed.

The gas forms a dissipative system that loses energy and collapses (within the first billion years) to a disk whilst conserving angular momentum.

Formed stars and clusters are essentially collisionless so the halo stars continue to have a spherical distribution, whilst more recently formed stars formed from gas already in a disk and so continue to be in a disk.