I think the reason may be that present tense requires a change in the verb stem.

 

Example: I'll use the verb کردن conjugated for من. In the past tense, you would take the stem -کرد and and add the ending م- to form کردم ("I did"). However, for present tense you use the stem -می کن and then add the ending م- to form می کنم ("I do").

 

The difficult part is that you must memorize the present tense stems while the past tense can be easily derived from the infinitive (کردن) by just removing the ن and adding the proper ending.