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Why is the third person singular conjugation different in the past tense?

Generally speaking, German verbs inflect with the following table

| Person | Inflection | Example |
|-|-|-|
| ich | -e | sag**e**, arbeit**e** |
| du | -(e)st | sag**st**, arbeit**est** |
| er/sie/es | -(e)t | sag**t**, arbeit**et** |
| wir | -en | sag**en**, arbeit**en** |
| ihr | -(e)t | sag**t**, arbeit**et** |
| sie (Plural) | -en | sag**en**, arbeit**en** |

The past tense (Präteritum) follows the same pattern with a modified stem, *except in the third person singular*.

| Person | Inflection | Example |
|-|-|-|
| ich | -e | sagt**e**, arbeitet**e** |
| du | -(e)st | sagt**est**, arbeitet**est** |
| er/sie/es | -**e** | sagt**e**, arbeitet**e** |
| wir | -en | sagt**en**, arbeitet**en** |
| ihr | -(e)t | sagt**t**, arbeitet**et** |
| sie (Plural) | -en | sagt**en**, arbeitet**en** |

Is there a particular reason that the third person singular conjugation is different in the past tense than in the present, or is this just some quirk of the language?

I would suspect a phonological reason (e.g. that having -et added to the already existing -t past tense marker is hard to pronounce), but "arbeitetet" does exist in the second person plural conjugation, and that has three(!) Ts in a row.