Existence is a funny term in Buddhism. The end goal of development stage practice is the recognition that the deity is inseparable from one’s self because the nature of any given deity is equal to our own in an absolute sense. After all, everything is empty of inherent existence and are all self arisen manifestations from a primordial ground of being, hence no you and no I as separate.

So some people do see them as external things to ask things of but others see them as mind created and used as supports for meditation to realize our own true nature. Depends on lineage and teacher.