Because you signed a contract to abide by the outcome of the arbitration proceedings (i.e., it's "binding"). If you appeal, the judge will toss it out and you'll have wasted your money.
This is not really obvious if you’re not from the US. In Poland (and I guess other continental Europe countries) you can appeal to a regular court. The possibility to appeal to court is considered one of the basic rights one has.
that should be illegal, it should be impossible to waive a basic right. this will affect poorer people much more, as they have less experience, money to invest into a good lawyer and job options. it's not a fair process.