Another reason I think is that D&D 1e was a lot deadlier to PCs than later editions, I think I can count on one hand the number of my PCs that lived past 7th level, most were killed long before 7th, but I seem to remember those that survived the early levels hit a wall at 7th level on lots of adventures.
As with so much about 1e, there is no single experience of the game. I thought much as you until I had set in on the tables of more DMs.
How deadly 1e is depends on a lot of things.
1) What method of character generation is provided for? Depending on which approved method you used, the methods would generate on average anything from barely survivable ordinary persons to veritable demigods. A fighter with 18 str or 18 con (or both!) roughly twice as powerful as one with ordinary stats, such that a 5th or 6th level fighter with two 18's is about worth a 10th level fighter without either.
2) How much treasure is provided, particularly with regards to magical items? The amount of treasure that the game provided varied even within its own guidelines, depending on which guidelines were considered definitive. In particular, the sort of random treasure provided by treasure type in the monster manuals, the random treasure in the appendix on dungeon generation, and the amount of treasure in published adventure paths (GDQ, for example) varied greatly. If you played with the monster manual being definitive, about 2/3rds or less of the XP a player would earn would come from treasure and magic items would be quite rare and extraordinary finds. If on the other hand you took GDQ or other published modules as definitive, each adventure was intended to provide sufficient treasure to ensure at least one and sometimes two levels were gained in the course of play, magic items would be plentiful, and treasure would be often 90% or more of the earned XP.
3) How closely were the rules adhered to? This was of particular importance with regard to spellcasters. If spellcasters weren't handicapped by application of the rules on casting time and so forth, they'd quickly become very powerful indeed.
4) How RB was your DM? Particularly as the party reached 10th level or so, and particularly if they had as much advantages as the game allowed, the game sort of broke down. Consider that the monsters in the game were rated on a scale of I through X, and that this rating corresponded to the average dungeon level they'd be encountered on, which in turn corresponded to the average level of the PC expected to reach that level. In order to provide continued challenge at 10th level and beyond, a DM had to be tactical, inventive, and have a degree of system mastery to choose and abuse the few foes available that could provide challenge to high level characters.
5) Are we talking before or after the Unearthed Arcana? The classes, class options, spells, and items in the Unearthed Arcana - to say nothing of probably the second most potent means of character generation if it was employed - significantly increased the expected power level of low level characters, effectively boosting the party by a couple of levels compared to pre-UA parties.
In the extreme case of a DM running a very tight ship, employing chargen of 4d6 drop 1 and watching it to prevent cheating, giving out treasure only as indicated by the treasure types (and even then putting a finger on the scale to stop potent items from appearing at low level), fully leveled up as a RBDM, and not running Unearthed Arcana, 1e was very deadly indeed. In the opposite case of a DM allowing generous chargen, generous magic item placement, UA rules and limited or no restrictions on spellcasters, little tactical acumen and conscious avoidance of 'unfun' or 'unfair' monsters (such as those with energy drain), 1e AD&D was really only particularly deadly at 1st level (and then only with no house rule that allowed maximum hit points at 1st level).