That's a trade-off you make to get both macro focusing and the focal length you want in the same lens. Most current-generation macro lenses, including Tamron's newer 90mm f/2.8, are internal focus (or "rear focus") lenses; they'll let you go to a 1:1 magnification, but they do that by significantly shortening the focal length of the lens while leaving the front of the lens anchored in space. The current-generation Tamron 90mm macro is only a 60mm lens at 1:1 magnification.
In the traditional unit focus layout, you focus closer by moving the lens, as a unit, away from the sensor (or film). To focus at 1:1, you have to move the lens a long way. Look at where the fron element is in relation to the outer barrel of the lens when the lens is focused at 1:1 -- that extension distance has to come from somewhere, and that "somewhere" is the length of the outer barrel of the lens. (Yes, it can be done by using two tubes, nested, which is far less stable, or by reducing the amount of overlap between the tubes when the lens is fully extended, which is also far less stable.) In the original version of this lens, a manual-focus version with an f/2.5 maximum aperture and the Adaptall 2 mount, the inset of the front element wasn't as deep (it still had what amounted to a built-in lens hood, just not nearly as deep), it could only focus to a 1:2 (half life-size) magnification without using an additional extension tube.
Again, you can go for an internal focus design. Just be aware that you'll need a 150mm IF macro to get the same working distance as you get with a 90mm macro that stays a 90mm macro throughout its focus range. If you use a 90mm IF lens, your working distance is that of a 60mm unit focus lens, so the lens element may sit right at the front of the lens, but the lens barrel is still going to be in the way of the light.