My guess is because they didn’t intend to be careless; they were just incompetent and seriously thought they could get away with it.
This is the difference between the Russians and the Saudis; the Saudis thought a “Get Smart” plot would be enough to let them get away with this, and their use of a body double (no matter how ludicrous) suggests both a degree of planning and a wish to conceal. They thought if they tried hard enough there’d be little suspicion.
Putin, on the other hand, is never seeking plausible deniability - what he’s actively working towards is inplausible deniability. Novichok? Plutonium tea? Might as well strangle your enemies with a Russian flag and leave it next to the body. Putin is the blood-covered farmer standing in a yard surrounded by headless chickens holding an axe, claiming “Was fox. Cunning fox.”
