Because in order to "ramp up their security", they'd need more users which makes this attack harder. Security isn't like a knob that you turn.

The security of proof-of-work based blockchains (which most of them are) is based on the assumption that the majority of participating nodes (or rather miners) as measured by the work they do, i.e. computing power, are honest.

Attacking a big, established blockchain like Bitcoin would require you to get more mining power than the rest of the network, which means massive datacenters of special purpose hardware.

Attacking a small blockchain still requires you to get more mining power than the rest of the network, but since there are not too many miners, you can rent enough mining power to have more than the rest of the network for an affordable price.

In the process, the attacker will legitimately get some currency, which is supposed to incentivize them to not actually attack the network, because it will affect the value of the coins they got − that's it, that's the only protection.