Why can't an endowment policy (Postal Life Insurance) be treated as a long-term investment?

PLI endowment policy gives more bonus compared to private endowment policies including LIC, then why can't a govt employee choose it as a long-term investment plan. I guess a term policy will suffice purely for insurance purpose for life. Can I treat PLI endowment policy as a long-term investment for future (not TERM POLICY/ TERM PLAN policy)?