Why KOH can absorb CO2?

All group 1 hydroxides are alkalis. As potassium (K) and sodium (Na) are in group 1, hence, KOH potassium hydroxide and NaOH sodium hydroxide are alkalis.

CO2 is an acidic gas.

When an alkali reacts with an acid, neutralization occurs and produce salt and water only. (By the definition of neutralization.)

Chemical equations:
2KOH (aq) + CO2 (g) → K2CO3 (aq) + H2O (l)
2NaOH (aq) + CO2 (g) → Na2CO3 (aq) + H2O (l)

As neutralization occurs, CO2 is absorbed by KOH or NaOH.