Because Bitcoiners strongly believe that Bitcoin will be adopted by more users who see a need for Bitcoin's properties (citizens experiencing significant currency debasement, institutional users who require fast finality, users trying to circumvent capital controls, ...) combined with the choice to maintain a limited blocksize, there is a strong belief that transaction fees will become prohibitive in the future, excluding small users from ever using on-chain transactions. The hope is that Layer 2 solutions will be ready by then.
A common argument presented about the fact that spam induces high fees is that we all knew that fees were always going to be high, so high fees shouldn't be an issue. 

This is argument is flawed for two reasons:
1. High fees are not the issue when it comes to spam, the issue is about the wastefulness and abuse of the network's functions that cause harm to the network as a whole.2. The reason why fees are high is obviously important. The argument that fees were always going to be high assumes that there will be a strong demand for Bitcoin's properties. When spam is the reason why there are high fees on the network, that is obviously not the case. The high fees are not what matters, it's the adoption and demand for Bitcoin's properties that causes them that's important.
This second point can easily be understood with a simple thought experiment:
● Let's assume that a nation-state level attacker funds an attack where they fill blocks with transactions sent back to themselves, consistently paying fees high enough to ensure that their transactions will be in the next block and stopping anyone else from transacting.● In this case, the fees will necessarily become high, but this situation will stop anyone else from transacting, stopping any real demand for Bitcoin's properties. The fees are high, yes, but would you still support a mitigation scheme to limit the reach of such an attack?