You can ask the government that question, but those are the rules.
I think I know the basic answer, though. Tax deferral has value (a cost to the public purse), and the government doesn't want to grant an unlimited tax deferral once withdrawals start. It's not in the public's interest to subsidize bequests, basically. Rules like this one are pretty common around the world. For example, the U.S. has Required Minimum Distributions (RMDs) on its Traditional 401(k)s and Traditional Individual Retirement Accounts (IRAs). Withdrawals must start no later than age 70 1/2. Roth accounts are different and don't have RMDs.
The single premium life annuity exception to the 10 year withdrawal rule is something the government explains. The government wants to encourage lifetime retirement financial security, for many good public policy reasons, so they grant that particular single exception.