The objection called tarjīḥ bilā murajjiḥ does not say that an event is impossible whenever its cause is unknown to an observer. An unknown explanation is different from no explanation at all.
The argument maintains that if a thing's own essence does not necessitate its existence, its existence cannot be inferred from that possible essence alone. A determining explanation is then sought.
“The lamp is lit” describes a fact; it does not mean that the lamp must always be lit. We examine the relevant conditions of its present state. The text extends this explanatory structure to the world's existence.