In a perfect world, traditional network firewalls could inspect and drop unwanted traffic, but every firewall has finite capacities. If determined enough, attackers can generate enough noise and make it impossible for legitimate traffic to pass through.
Unmanaged devices are susceptible
Accepting inbound network traffic leaves a front door open for attacks. Flaws in applications and operating systems can be exploited by attackers.
Artificial chokepoints cause latency
Traditional methods of inspecting traffic and mitigating attacks often result in unwanted performance tradeoffs, slowing down applications and impairing end user experience.