It’s usually not, but you’re looking at the very low end of pricing on servers which means that if they scale additional IP prices to the same level that they scaled that first IP + server, they’ll run out of IPs and have servers leftover, and no profit. You might get a VPS + IP for $1.99/m but selling you an additional IP for $0.01/m isn’t profitable and they’re only making this all up by scale, a scale they won’t reach by basically giving away the IPs. Basically cheap servers are a very calculated move, and if you start selling off pieces of those calculations individually for pennies you’ll break the business plan.
If you’re looking above budget level at more “normal” prices you’ll find IPs to be much cheaper than the servers. Think of someone charging you $50/m for a VPS and $1/m per IP, that IP becomes a fraction of the cost of the server. If they’re discounting the VPS heavily they can’t afford to drop all of the IPs into the heavily discounted server for such an equivalent low price.