Because it's the proper conjugated form of the verb in this case.
I read - ik lees
You read - jij/u leest
He reads - hij leest
We read - wij lezen
You (plural) read - jullie lezen
How do you go from 'lezen' to 'lees(t)'? Why do you lose the 'z'? Is that normal or is this verb irregular?
It's something u have to memories but can't be considered as irregular verb