Why not? I'm assuming that the whole point of making a sequel is to make a game with more similarities than differences to a previous one. I might like a sequel more but not enjoying one game in a series is a fair indication that I won't like the others either. So why should I insist trying when there are so many games to play?
Fair enough, but sometimes a sequel makes the experience better as a whole (this doesn't happen as much anymore, but it used to be that sequels were improvements upon the foundation started by the original game). But playing just one game in a series - especially if it's just the first game - isn't really an indication of how well the other games in a series fare after the developers had a chance to review what worked, what didn't, and what improvements/changes needed to be made. The most recent example of this that i can think of off the top of my head is the transition from Oblivion to Skyrim - Skyrim is a vast improvement over Oblivion in several key areas, and judging Skyrim by only playing Oblivion (or even Morrowind) wouldn't give you a very good indication of how the newest entry plays, aside from basic similarities (open-world exploration, a large continent to traverse, faction quests, single character, etc.)
Our biggest difference here is that I don't anymore think that gamers should be punished in whatever way. If you fail at being stealthy you should either be allowed to try again (preferably in a way that would not spoil the atmosphere like saving, reloading and acting with previous knowledge does) or, ideally, there should be a stealthy way to correct your mistake. Switching from good stealth gameplay to bad combat gameplay is not something that I find appealing anymore.
I'm not sure it's possible to make a stealth experience challenging and rewarding at the same time without some form of "punishment" for failure; the tension that slowly builds while sneaking through a level is one of the keys for an immersive stealth experience, and without it, sneaking feels pointless. The newest Splinter Cell is a great example of this: while the "classic" Splinter Cell titles (1-4) made combat a risky proposition and movements needed to be slow and precise in order to successfully advance through a level, the newest entry moved far more towards an action approach. The loss of tension from knowing that failure would mean getting in a tricky situation made the stealth gameplay rather unsatisfying. Running away or using the tools at your disposal is always a way to get out of trouble in "pure" stealth games versus direct combat.
Please keep in mind that I have no intention to bash the game. I can see the good design and I understand why it's considered a classic. I know it took a fresh approach and that inevitably results in rough edges and I understand it focused on a unique type of gameplay that is likely to have improved a lot by other games since. I'm just registering some thoughts about the game as I see it on its own more than 14 years later and, inspired by it, some thoughts about games in general.
Don't get me wrong, if you don't like the game then you don't like it, and that certainly doesn't make you "wrong" in some way - I'm not going to pull a Fluent and try to force you to think otherwise
. Like most games, Thief isn't for everyone, but it's highly rewarding for those who do enjoy that style of gameplay. I just wanted to suggest playing the sequels because they might be more enjoyable for you than the first game; I feel that Thief 2+3 are much better overall than the first game, but that's just my opinion.