lowercase() function in python

There is no lowercase() function, so your question is a little confusing from the get-go (str is not a function either, it's a class you construct, but it's very function-like, so I get the confusion there).

Python doesn't provide a top-level lowercase function because it's not a common interface many types need (str, bytes and bytearray are the only built-ins that support case changes to my knowledge), so rather than making it one of the special methods with built-in support, they leave it as a simple named method of the types that need it:

mystring = "mIxEdCaSe"
lowerstring = mystring.lower()  # Produces "mixedcase"

The ability to use str to perform the conversion, then use methods to manipulate the resulting string, avoids dumping hundreds of functions in the built-in namespace that just mean "call str, then this method on it" and makes the functionality more discoverable (once you have a str in the interactive interpreter, you can type mystring.<TAB> and see the available methods, rather than looking at hundreds of built-in functions, most of which would have nothing to do with strings).

As a side-note: There is one built-in that combines class construction and calling a simple named method on it: sorted, which is exactly equivalent to a function like:

def sorted(iterable, /, **kwargs):
    newlist = list(iterable)
    newlist.sort(**kwargs)
    return newlist

It exists mostly because that particular action (converting some other iterable to a list and sorting it) is performed so frequently (in my experience, I call sorted at least 10x as often as I .sort() a list in-place, because I'm usually using a "better" data structure for the task up until I need it sorted) that it was worth the built-in namespace pollution. But it's the exception, not the rule; for other cases, if you can do it with a class constructor followed by a method call, they expect you to do that.