Think of it. TO HAVE something is static. So, in a binary sense, you have something or not. If they, you are in the process of obtaining or getting something, there is the sense of "busy with it" that the progressive tense speaks to.
HÊ > KRY
Ek het 'n pen. (Present Tense/Hede)
Ek kry* 'n pen. (I am getting a pen.) Progressive
Ek sal 'n pen kry. (I will be getting a pen./I will get a pen.) Future Progressive
Ek het 'n pen gekry. (I got a pen./I was getting a pen.) Past Progressive.
*KRY = GET
The lesson. HET, not as a past tense marker, but as a verb of possession is not progressive. It indicates something static, just like WEES indicates something static. With WEES, you use WORD to indicate change (progression) and with HíŠ, you use KRY to indicate change or progression.