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There are several reasons.
Active sites are usually buried in the middle of enzymes where they are relatively sheltered from the exterior environment. This is necessary to keep the reactants from forming inappropriate bonds and adopting inappropriate conformations which they would if they were exposed to more variable forces. The enzyme depends on hydrophobic interactions between interior amino acids to fold and maintain its conformation (see entropic binding energy) so most of the amino acids around the active site are hydrophobic.
A subtler reason is the fact that the products, reactants and transition states have different structures; often the enzyme experiences a change in conformation through the reaction too. The hydrophobic amino acids have to be able to accomodate all those different conformations and the many, relatively weak bonds they form are more suited to this plasticity than the fewer, stronger nonbonded interactions which are usually formed by hydrophillic amino acids.
It has been a long, long time since someone asked a proteins question on here!
thank you sooo much, i have to explain it tomorrow to my class and i did not remember very solid reasons