This is because in order to get a good espresso, there is a certain extraction formula that need to be apply on the coffee beans. And it needs to have a pressure of no less than 12 bars.
For espresso machines that are less than $100, being all steam driven machines, they cannot achieve this level of pressure, so the espresso that they extracted would not be able to meet the standard of a real espresso.
So, what is a real espresso? Some call this an experience, but by sight, it must be topped by a slightly golden color crema to be consider a good espresso.
By the way, if you asking for what makes a good espresso, then you are probably just starting to make a brew yourself at home, if that is the case, my personal take is start with a steam driven machine to assess if you like brewing coffee at home.
Out of all the steam driven machines, the one that would make a decent coffee espresso and allows you to have a feel of what espresso making is like would be the Delonghi EC 5.
Retailing for less than $40, this espresso machine is in fact cheaper than some programmable coffee maker. So, this is a perfect machine for beginner or folks that are switching from drip filter coffee maker.