Why my job expects too much from me?

I am one of the newest employees. And they sent me constant emails to discuss my work or to ask me to volunteer to learn new things to help them. However, other employees who have been here for 20 years are not pressured to learn any of these. I am the first person they come to when they need a shift coverage. I have only been there for six months. They also expect me to run the department in the weekends with other experienced employees until they find a senior employee to fill the position. I dont know why they expect too much. Even though they are not always willing to provide me with the training, support or tools to preform. My email to ask for tools get ignored for months. My training was lacking I was placed and moved based on their needs not mine. and I was expected to preform complicated tasks within my first three months I got emails asking me if I did them yet. these tasks are complicated employees with 20+ years of experience still dont know how to do. Why they put this pressure only on me?
Why my job expects too much from me?
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