I assume everyone here already knows this and why. But for the benefit of the court, here are the reasons:
First, liches have an LA of 4. High LA is practically as a rule unworkable. Especially on a spellcaster, as they lose more by missing out on actual class levels. Here it means no 9th level spells until Epic, which just sucks. It's not even that good an LA 4. Minor stat and skill gains, and the whole respawning thing is done better via other, cheaper means.
Second, it's a double whammy penalty as you also have to pay 120,000 gp and 7,200 xp. You won't even have 120k gp at the minimum level you can become a lich (11) even if you sold everything you had and somehow got full price for it. You aren't doing it until later and even then it's still a sizeable chunk of your income. The XP cost comparatively isn't that big a deal. See: Riding the Gravy Train. Though the LA still fucks with it.
Ok, so now you're a lich. Great. You get some immunities... and you get some new weaknesses. At this level vs intelligent opponents your weaknesses matter more than your strengths. The biggest one is that an equal level cleric who hasn't optimized turning gets a coin toss to render you useless (turn) or their bitch (rebuke) with no save. The turn resist +4 offsets the 4 LA, leaving you counting as an undead with a HD = to your character level. And if they have Greater Turning? Sucks to be you. Also, Disintegrate, and you might actually lose HP here. You get the idea.
There's also a third one. By RAW you have to become evil. Now I don't mind that, but some DMs might and some players might. It's rather limited in what games you could do it in even if it didn't suck.
So now the question. How to make this cool concept not suck horribly in execution so it might actually be worth doing?