Because disputes are expensive, time-consuming and generally not fun
An ambiguous term is a dispute waiting to happen. Far better to resolve the ambiguity and correctly allocate (and price) the risk at the start of a contract when nothing is at stake then have that term be at the centre of a dispute with $10 million on the table that both parties think is theirs.
At best, a dispute strains the relationship, at worst, it involves time and costs putting the matter before an arbitrator or a judge and hope the resolve the ambiguity your way. I prefer to play roulette at a casino not a courtroom.
Contra preferentum is unlikely to be a successful argument in any form of non-determinative dispute resolution - essentially the proffering party is never going to agree that it applies.
Further, contra preferentum is a last-resort resolution mechanism. Maybe what you read as ambiguous is actually clear. Or clear in context, or clear when industry norms are considered or when past dealing or current dealings between the parties are considered. Relying on it is a real long shot.
Also, almost all B2B contracts explicitly exclude contra preferentum as a method of resolving ambiguous terms.