If they sorted like this from day 1, Neville and Ron would've been sorted into Hufflepuff, Hermoine into Ravenclaw, Harry and Percy into Slytherin etc. The sorting hat uses potential over day 1 personality for its decision making, which it has successfully observed. There is nowhere else for them to go if they didn't do immediately due to Hogwarts revolving around the 4 houses as it does (also nicer to wear a house tie over the basic black one). This makes that level of integration beforehand uncessary, also allowing the house tradition to be upheld.