We look at the definition of a sacrament: (1) a sacred sign (2) instituted by Christ (3) to efficaciously give grace (4) and so build up the Church. Each dimension is essential.
There are three parts of all the sacraments: the "sacrramentum" (the visible sacrament that the Church celebrates), the "res" (the inner grace that is put into the soul), and the "res et sacramentum" (the enduring element of the sacrament that allows the grace to be accessed in the future).
The sacraments are fitting because of two primary reason (among others):
1) God has always reached out to us through physical signs (notable the humanity of Jesus Christ). Why should we expect to see a radical shift of the divine strategy, especially after he doubled down on it in the humanity of Jesus Christ? It's fitting we have tangible access points to contact God.
2) We're sensible, physical beings. Everything that we know and love first comes to us through the senses. God can do what He wants, but it is fitting that He loves His human creatures in a way proper to how we are created: physical, tangible, sensible, corporal.
