Why should 'obejśc' be an intransitive verb once it needs a direct object
I agree. Transitive is as transitive does: it takes a direct object so it is transitive. No amount of irregularity in the verb itself can change that.
However, not all transitive verbs in Polish form passive participles, for example
mieć and woleć, and obejść falls into this category. What is weird is that, as far as I can tell (maybe Emiwis could confirm)
obchodzony, from the imperfective form, seems to exist. Another weirdness is that unikać, uniknąć, to avoid, which take the genitive, and are therefore
intransitive, form the passive participles unikany, uniknięty. See Swan, p. 302: polish.slavic.pitt.edu/grammar.pdf