There’s a really common convention in MMOs that you can’t move around while casting a spell. It’s fine to have fireworks going off all around you, to be hit in the face with a giant hammer, to be tripping over your robes, standing knee deep in a river or watch your comrades being ripped limb from limb … but heaven forfend that you might take a sidestep to the right.
I’ve never been really sure where this came from, I think it’s a relic from a more wargamey past where caster units had their movement penalised to give melee a chance to catch up and kill them.
On the odd occasion when I’ve played a character that could walk and chew gum at the same time (aka cast on the move, or use lots of instant cast spells) it was great fun to be more mobile. Although it does your head in getting used to playing a class which can cast on the move and has a casting time. If you play a caster, would you like to be more mobile?