• Because $s^{0.25}p^{0.75}$ is a mouthful. – Zhe Dec 4 '17 at 19:39
As has been pointed out in the comments, an $\mathrm{sp}^3$ orbital is a linear combination not of one s orbital and three p orbitals, but rather of one part s orbital and three parts p orbital. Stated differently, the numbers in $sp^3$ denotes the ratio and not the number of orbitals we combine.
Oddities such as the $\mathrm{sp^5}$ orbitals in cyclopropane can therefore be rationalised. It does not mean that there are five p orbitals which form hybrids, which is obviously impossible. It simply means that that particular orbital has ~1/6 s orbital character and ~5/6 p orbital character.