Orthodox daily readings app?

I downloaded this on recommendation from this group and am loving it so far. I have shared I was catholic for most of my life. I always used the Laudate app as a catholic. I noticed the daily readings always have an Old Testament reading that typically tied into the epistle and gospel readings. Why does the orthodox app not have an Old Testament reading. I know the TLM liturgy does not either. Just curious as to why both traditions (the old TLM) does not include the Old Testament in the liturgy? Thanks