Because this is what would make YOU happy. Obviously, one would only have to look at this forum for proof, people feel a lot different than you do. Some want to just see the Warden, some don't want him there, some want to talk to him, and so on and so forth.
So, when you have millions of people wanting different things, and take into calculations that at least half of DA:O fans would be pissed about their Wardens not being included in the way they want them to be included, why would Bioware then put the Warden in? It would be silly to.
The problem is that the Warden deserves to be mentioned in more than some lame agent mini-mission
If you decide not to show them fine
Then make sth awesome without them showing-off. A series of quests, agent mission. Or even just people talking about the great Hero of Fereldan still being the frigging Hero of Fereldan when the country needs them
Which is what I wrote, since the Warden has no set personality its impossible to recreate the player's Warden. Unless the Warden appears as a line of shop dummies with no voice and personality its nigh on impossible to bring the Warden from DAO to DAI. Bioware would need to read our minds to be able to recreate our Wardens, it just isn't going to happen.
I'm not sure what you mean about Shepard he/she did have a set face and personality (sure you could modify the face) like Hawke making it possible to bring a character from one game to another.
No one needs to recreate the Warden. It's not the game about the Warden, or Hawke. There is no even a need (or possibility) to give them a personality. That's just a nonsense to even think of
A quest, battle, situation and - if the Warden just has to talk - neutrally-sounding lines. Not eveything just has to shout "aggressive/pacific/sarcastic". Like 40% of Shepard's lines in ME3 which were so neutral the whole three options we were given sounded like the same thing
I mean that despite Shepard had a voice, a personality (sort of) most fans didn't feel like it didn't match their Shepards. Even I managed to be able to play as a self-insert in a way and didn't feel disconnected from the PC. Unlike Hawke, whose voice wasn't neutral enough for this effect
E.g. go a see some fanart, read some fanfictions. There are crazy Shepards, silly Shepards, hilarious, angry, sad and many more