There are more bows in loot than almost any other weapon type in the higher level tiers. They are rather powerful, and don't need improving.
I find the vast majority of them rather underwhelming compared to a buffed melee weapon, and I think the lack of bow-using characters is a testament to that. That said, I think the problem is more prevalent at the sub-ensorcelled tiers.
As a quick mathematical exploration of the problem...
Decent longsword (because 1d8 base) at ensorcelled tier--Blade of the Heavens, 1d8 base, 1d6 sonic, 1d4 divine, 4 massive crits, +4 from enhancement
Ice Dragon Bow: 1d8 base, 1d6 cold (unlimited), +5 mighty, 2d8 massive crits -- I picked this because it is very much in line with several other ensorcelled bows I have seen.
Now throw in several factors:
1) elemental weapon on the longsword
2) the longsword's str bonus damage isn't limited by mighty, whereas a bow's is (and it's easy enough to get +6 on even base 10 str characters, which would exceed the ensorcelled bow listed above)
3) bows do not get a damage bonus from enhancement, melee weapons do -- and most of the bows I've seen only have a +1 or so along with whatever unlimited ammo bonus
But for the sake of being nice to bows in this argument, let's pretend the elemental damage bonus is 2d6 rather than 1d6, as I believe I have seen such at ensorcelled or the next tier, as well as +1 on the unlimited arrows. We're also going to be nice to the bow and assume a buffed base 10 str character is using both the longsword and the bow, rather than 14 or higher as would be common. We'll be even nicer and assume that the elemental weapon is only being cast by a 10th level caster (1d4+5 dmg), as per the scrolls you can buy, as opposed to say a 20th level caster (which would give the full 1d4 + 10).
Longsword damage calculation = 1d8 + 1d4 + 1d6 + 4 + 6 (str) + (1d4+5) -- min 19, max 37, avg 28
Bow damage calculation = 1d8 + 2d6 + 1 (unl. arrows) + 5 (mighty) -- min 9, max 26, avg 17.5
For lulz, longsword without EMW = 1d8 + 1d4 + 1d6 + 4 + 6 (str) -- min 13, max 28, avg 20.5
To review:
I was being generous to the bow in several ways from the outset, as detailed above.
Longsword already beats out the bow without elemental weapon, and demolishes it with EMW.
The EMW was only scroll level.
We were assuming a 10 base strength character using a longsword (unlikely).
Feel free to throw another +5 to all of the buffed longsword numbers above for full EMW, and and another +2-5 from strength for a typical melee fighter. These are bonuses melee weapons can easily attain that bow has 0 way to account for.
Now in terms of tactical options, bow allows you to engage (potentially) more safely from a range, or without introducing more clutter, but a 1 handed melee weapon introduces benefits of its own in the form of a shield--the shield not only provides extra AC, but often other statistical benefits. A two handed melee weapon benefits from 1.5x str bonus, and a 2 handed user will almost certainly be rocking more than the 10 base str assumed above.
In addition, the primary melee enhancing prestige class (WM) absolutely blows the primary bow enhancing class (AA) out of the water in terms of increased damage potential. Not that bow wasn't already at a disadvantage. All of the above is even worse when you compare sub-ensorcelled gear.
I congratulate you if you have an absolutely god-tier bow that can keep up with an equal tiered melee weapon statistically, but the fact is, bow needs help, as evidenced in the (utter lack of) player base and a simple numerical breakdown.
Edit: Just to have a little more fun with this, I'll give you numbers for an easily attainable longsword character (level 16, ecl 0) that has the same longsword with level 20 EMW cast on it vs a completely ridiculous fantasy bow the likes of which I've never seen on CD:
Character str: 32 (16 at lvl 1, +4 from levels, +6 from buffs) -- +11 modifier
Longbow stats: +2d10 elemental damage and +3 unlimited arrows, +6 mighty
Longsword calc: 1d8 + 1d6 + 1d4 + 4 + 11 + (1d4+10) = min 29, max 47, avg 38
Longbow calc: 1d8 + 2d10 + 3 + 6 = min 12, max 37, avg 24.5
Yeeeeeeeeeeeaaaaaaaa....