Why doesn’t the Catholic Church baptize as Jesus was baptised in the river Jordan?

Sounds to me like you’re referring to Baptism by immersion, where the person is completely submerged under the water. This is different from how Baptism is usually done, where water is simply poured over the head of the person being baptized. Actually, in the Catholic Church, both of these forms of Baptism are valid and can be used. Some countries or regions might prefer one form over another or have certain rules in place, but both forms of Baptism (by immersion in water and by pouring water) are perfectly valid, provided the Baptism is done with the correct words said and by a valid minister (a bishop, priest, or deacon, except in danger of death, in which case a layperson can validly baptize).

Most Catholics are baptized as infants and for these infant Baptisms, pouring water over the head is much more common (since it is much more practical for babies). Even for adult Baptism, pouring water over the head seems to be more common. I think this is generally for practical reasons, since doing Baptism by full immersion means more time and a spare change of clothes once the person dries off. In the early Church, however, Baptism by immersion was standard! Whether Baptism is done by immersion or by pouring water over the head, the significance of the sacrament remains the same.