This is a good question. The progressive tense is preferable in this example because the situation may change. We use simple present for facts, repeated actions, or for things that generally are true in the present time period. The phrase "work is improving all the time" indicates a problem with work performance that could recur.

The meaning here is: "your work continues to improve (over time)". Thus, we use the progressive tense.

esra.R, Your work improves all the time. This is a general statement of fact. It refers to something in the historical present and is a comment about the constant improvement of your work. Your work is improving all the time. This refers to the work that is going on now in real time. It means work that is in progress, even if you are not working at this exact moment.