Because you are weak

. Just kidding!! I am now a 46 year old woman and can do many, many more than I could do in my youth. Most women do not have great upper body strength. Not sexist comment, it's a biological difference between the genders.
I ran track and played soccer for years so my hams and glutes are strong as sh*t, but I really have added in a lot more weight training in later years and a UXF class that does many planks, burpees, clean&press, push ups, etc. It's amazing how much more strength I now have in my upper body and can knock out some serious push-ups--even the spider man type.
Practice. Practice. Practice. Start from your knees first if it helps. What I always did was try as many as I could from my toes until my form started to slide as I got fatigued and then I dropped to my knees. In the beginning this was just about 1

---but you'll gain quickly. Core strength is a big issue with push ups and planks help a great deal with that too.