Because contar is a stem-changing verb in the present tense. Its o changes to ue in most forms:
So cuento is the correct yo form.
Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.
Adding yo is possible, but it is usually only used for emphasis, contrast, or clarity.
In this sentence, hasta means up to or until, but in natural English we usually translate the whole phrase as count to eighty.
With numbers, contar hasta + number is the normal pattern:
So hasta is the word Spanish uses in this structure.
Because after contar, Spanish normally uses hasta when talking about reaching a number in a count.
So the standard expression is:
Not:
If you want to show a range, you might say something like cuento de uno a ochenta for I count from one to eighty.
Because when Spanish uses a number after hasta in this kind of counting expression, it normally uses the bare number:
Adding el would usually change the meaning and make it sound more like the number eighty rather than simply counting up to that number.
A good approximation is:
The stress is on the middle syllable:
In Latin American Spanish, the ch sounds like the ch in chair.
Spanish present tense can cover both ideas depending on context.
So cuento hasta ochenta can mean:
If you want to strongly emphasize the ongoing action, Spanish can also use:
But the simple present is very common and natural.