Because they need more money than banks are willing to lend, their present circumstances do not imply collateral/predictable revenues/etc which would justify an underwriting decision to lend more, and their implied failure rates would make lending at commercially acceptable interest rates a prohibitively expensive proposition for the bank.
For example, take stock of two guys in their mid-twenties with $30k of their $40k combined net worth tied into a business. The business has proven revenues of $10k. They seek $2 million in capital to expand to their next milestone. No bank in its right mind will loan them $2 million -- they're literally incapable of making the interest payments on it, to say nothing of predictably paying it back. By comparison, a VC firm might assign a notional value to their business of $3 million (the bank would say "It's, literally, worthless: liquidation costs swamp accumulated value of capital."), put in $2 million and own 40% of the company, and see 10 similarly situated investments fail totally to sell this one company for $200 million 5 years from now.
That's the important point. Lots of VCs aren't really investing in a company, but are aiming to have invested in the next Google or other large tech company. They are playing blackjack.
A bank is not your business partner. If you fail via a bank loan, the bank still wants their money AND interest.
If your business goes bankrupt, the bank only gets their money if they required you to personally cosign for the loan. Of course, if your business isn't profitable yet, it's likely that they'll make you do exactly that, as a condition of getting the loan in the first place -- especially for tech companies that don't have many assets that can be liquidated in case of failure.
For this reason - as well as other reason people here already mentionned - the banks won't lend so much money to a young tech company.
Banks do not have as much vested interest in a startup as VCs. So their risk appetite is much lower than VCs. VCs lend money based on future potential of making it big and they take a lot more risk than a usual lender/bank.
Thats just the money part. VCs always provide a network of connections and key contact (mostly) who can also help your startup. Good VCs will provide mentorship and valuable help.