Transaction costs are very significant. As soon as a wallet contains money it is save to consider it as real. Distributing your money to multiple wallets as a single owner is very costly and makes no sense.
2. Using a new address per Tx is one of the only ways you can preserve anonymity.
3. Lots of people have more than one wallet.
4. Manipulators could automate the creation of wallets at essentially no cost.
5. The minimum value of bitcoin is 1 satoshi which is 0.00000001 BTC or 0.0000398886 USD at current prices.
To put that in perspective you could fund approximately 250 wallets with 1 satoshi each for one penny.