It’s because the animations are unique. Some characters do actually have unique bound weapons and animations that can be altered with new weaponry. For example, kicking jesuses have a unique weapon and animation but can be equipped with weapons, 6 Star a new threat Dwight has a replaceable bound weapon even though the crossbow bolt is a unique animation, and numerous michonne versions have a special stance for katana weapons.
That said, making additional animations takes a lot of extra artwork, time, effort, and balancing concerns. With artwork, it’s important to remember that every moving and non moving appearance for each component of a given character is a seperate art piece entirely. Every single frame of an animation in which an object changes its shape, angle, etc requires new art. Doing the animations for both a bound weapon and a normal attack means at least twice the animation work, whereas the artist could instead just get two different characters drawn up in that time. Oftentimes, these bound weapons act with the characters in a specific, intentionally designed way, such as Theresa’s stun attack working with her multiple-attack rush.
Bound weapons may be a PITA at times, but they’re here to stay in all but the most special of circumstances.