Because the answer doesn't matter on individual transactions. Your Realtor will have lots of statistics on the average spread--which may consist of dozens or even hundreds of transactions. And, depending on the market, it's often 2%-6%, though it can be more or less. But, as a home buyer, it won't help you to know that the average $300,000 listing sells for, say, $288,000. That's because you aren't buying hundreds of homes. So if you use that general ratio (in this case, 96% of the listing price), you could end up substantially overpaying. Or you could be overlooking the bargain of the year. The real question is: What is the house worth? NOT "what is the house listed for?"
Suppose you have 3 homes, each priced at $300,000. The first is an absolute bargain. It's really worth $330,000. In that case, it might actually make sense to offer more than the listing price. (Your Realtor can advise you on strategy.)
The second house is fairly priced at $300,000. You might offer $300,000.
The third house is overpriced. It's really worth $275,000. Using our example above, if you went with the averages and offered $288,000, you'd be overpaying. The house might not appraise for what you'd offered. And if it did, you'd really be starting out upside down.
One other point: Asking price and sold price don't tell the whole story. Often, buyers will ask for seller concessions or subsidies--for instance, to help with the closing costs. In our example above, let's say the house is priced at $300,000. And you see that the average sold price is $288,000. Well, the "average" seller may not really be receiving $288,000. That seller may be giving 1%, 2%, or even 3% to the buyer to help with closing costs. The seller might really, truly be netting $283,000. Now, if you look at the MLS statistics--and even if you go down to the courthouse and look at the sales transactions--it'll show a sales price of $288,000. But that's not the real price the seller accepted.
So, again, you're asking the wrong question. The right question is: How much is the house actually worth?