Because serverless 1) uses pay-as-you-go more often than traditional models 2) scales better to achieve massive usage. If you DDoS my $5/month DigitalOcean VPS

Because serverless 1) uses pay-as-you-go more often than traditional models 2) scales better to achieve massive usage. If you DDoS my $5/month DigitalOcean VPS, it's not going to increase how much I have to pay DigitalOcean in terms of compute, and most likely the VPS will buckle under load before it can actually rack up massive transfer costs (since those are also pay-as-you-go for DO).