Because the math checks out.
For a high level description, QEC works a bit like this:
10 qubits with a 1% error rate become 1 EC qubit with a 0.01% error rate.
You can scale this in two ways. First, you can simply have more and more EC qubits working together. Second, you can near the error correcting codes.
10 EC qubits with a 0.01% error rate become one double-EC qubit with a 0.0001% error rate.
You can repeat this indefinitely. The math works out.
The remaining difficulty is mass producing qubits with a sufficiently low error rate to get the EC party started.
Meanwhile research on error correcting codes continues to try to find more efficient codes.
That’s a good point which is part of why there is a lot of active research into quantum networking. Once you can connect two otherwise independent quantum computers, you no longer have the issue of increasing crosstalk and other difficulties in producing larger individual quantum chips. Instead you can produce multiple copies of the same chip and connect them together.